The project herein was supported by the National Science Foundation Grant Award No. EEC- 0808716: Dr. Richard L. Zollars, Principal Investigator. This module was developed by the authors and does not necessarily represent an official endorsement by the National Science Foundation. Using Microbial Fuel Cells in the High School Science Classroom Lisa Swanson Clarkston High School Clarkston, WA Jessica Schultz Culdesac High School Culdesac, ID Washington State University Mentors Dr. Haluk Beyenal Chemical Engineering and Bioengineering & Hung Nguyen Graduate Research Assistant July, 2008
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The project herein was supported by the National Science Foundation Grant Award No. EEC-0808716: Dr. Richard L. Zollars, Principal Investigator. This module was developed by the authors
and does not necessarily represent an official endorsement by the National Science Foundation.
Using Microbial Fuel Cells in the High School Science Classroom
o The difference in energy potential between two substances (i.e. zinc and copper)
based on their ability to give up electrons.
o The amount of electricity in the form of electrons passing through a substance (ie.
along a wire or cable). Measured in volts.
Current-The rate of flow (speed) of electricity (electrons) through a substance (ie. along a
wire or cable). Measured in amps.
Ohm- The measurement of resistance a substance has to electron (electricity) flow
(insulators have greater resistance, higher ohms, to electron flow than conductors).
Conductivity- How readily a material allows electrons (electricity) to pass through it.
Electrode – Either of two posts by which electrons (electricity) enters or leaves a battery.
Anode- Also known as the positive post. The post that, through chemical reactions,
produces protons (H+). The protons will pass through the electrolyte to the negative post
(opposites attract).
Cathode- Also known as the negative post. The post where protons will combine with
electrons.
Electrolyte- The material the electrodes are contained in. The electrolyte allows the
protons to pass to the cathode so as to complete the circuit.
Serial battery arrangement- Connecting a series of batteries in such a way so as to
increase the voltage output without increasing amperage. In a serial arrangement the
negative post of one battery is connected to the positive post of the next battery.
27
Parallel battery arrangement- Connecting a series of batteries in such a way so as to
increase the amperage without increasing voltage. In a parallel arrangement the negative
posts of the batteries are connected together as are the positive posts.
Key terms crossword follows as an assessment tool/review worksheet.
28
Battery Basics
Battery Basics
29
Across 2. measurement of resistance a substance has to electron (electricity) flow. 5. collect on this terminal of the battery. 8. difference in energy potential between two substances (i.e. zinc and copper) based on their ability to give up electrons. 10. that have a greater resistance. 15. material that allows the protons to pass to the cathode. 16. rate of flow (speed) of electricity (electrons) through a substance. Down 1. stacks of zinc, saltwater soaked paper, and silver that generates a voltage. 3. type of battery arrangement the currents add up. 4. Speed of electron production by the chemical reaction within the battery. 6. that have a lower resistance. 7. type of battery arrangement the voltages add up. 9. like a flashlight, radio or cell phone that you connect to the battery. 11. of battery used in automobiles. 12. of battery like Duracell or Energizer, used in flashlights. 13. chemical reaction that produces electrons. 14. positive post on a battery.
30
Activity #3
Power point presentation introducing cellular respiration and microbial fuel cells
Purpose
The purpose of this activity is to introduce students to cellular respiration and how we use
cellular respiration to generate energy in a microbial fuel cell.
Prerequisite Knowledge
Students should have an understanding of energy and how energy transfers through a system.
Instructional Strategies
The teacher should familiarize themselves with the background information in Appendix 1 in
order to understand Microbial Fuel Cells and their operation. While presenting the PowerPoint
Presentation, student should take notes on cellular respiration and MFCs.
See attached PowerPoint File
31
Activity #4 Assembling the MFC
(Adapted from Appendix 1)
Purpose
The purpose of this activity is to assemble the MFC
Prerequisite Knowledge
Students should have a basic understanding of microbial fuel cells and their components.
Instructional Strategies:
The teacher should observe students and help as needed. Each member of the group should
participate in the assembly of the MFC, whether it be handing various components to the
assembler or reading the procedure to the assembler.
Data Collection:
None needed for this activity.
Data Analysis:
None
Evaluation Protocols:
The teacher should evaluate the group members on their cooperative efforts in assembling the
fuel cell. The assembled fuel cell could also be an evaluation component. Questions regarding
fuel cell components follow as a handout to students.
Worksheet/Handout to be Given to Students: (on next page)
32
Equipment for MFC
Components of a MFC
(a) Anode and cathode compartments. The holes at the top are used
to insert electrodes or to make electrical connections to electrodes in
addition to inserting a reference electrode.
(b) Anode and cathode cover plates. The anode and the cathode
compartments and the cover plates are fabricated from polycarbonate.
The working volume of each chamber is approximately 100 mL.
(c) Cation exchange membrane (C-7000). The anode and the
cathode are separated by this cation exchange membrane.
(d) Graphite electrode. Graphite is used due to its inert structure.
We use graphite as both anode and cathode.
(e) Air electrode. An air cathode composed of Pt wire mesh and
coated with carbon powder is used. The choice of cathode material
depends on the oxidizing agent used for the cathodic reaction. When
oxygen is used as an electron acceptor, carbon materials are used with
Pt or Ni catalysts because plain carbon gives a high kinetic limitation.
(f) Rubber gasket. A minimum of four gaskets are required for
sealing one MFC.
33
(g) Bolts and wing nuts. Ensure that the structural hardware used to
assemble the MFC is 316L stainless steel. We use 316L stainless
steel due to its resistance to corrosion. During the experiments these
parts will be wet. If we use a lower grade of stainless steel it corrodes
easily.
(h) Silicon adhesives. Silicon adhesives are used to seal any unused
ports on the MFC.
(i) Conductive epoxy. Conductive epoxy is used to secure electrical
connections between wires and electrodes.
(j) Fittings. The fittings are used to connect silicon tubes to the inlet
and outlet.
(k) Barbed tube connectors. These are used to connect two silicon
tubes.
(l) Saturated calomel electrode (SCE). We use this as a reference
electrode.
Figure 3 List of parts used to assemble a MFC (From Appendix 1)
34
Equipment for operation and maintenance of MFC
(a) Silicon tube. Various lengths of tubing are used for the feed and waste
streams.
(c) Clamps. These are used to close ports of inlets and outlets of the MFC.
(f) Syringe: The syringe is used to inject water into the MFC prior to
autoclaving. It is also used to inoculate the MFC with yeast and feed the
sterile growth medium into the MFC.
(h) Flask with growth medium. A growth medium suitable for the given
microorganism is prepared. It is essential to properly mix the medium for
optimum growth.
(i) Tin foil for covering flasks
(j) Flask with inoculated yeast. The yeast are nourished before the MFCs
are started in order to decrease the lag phase.
Figure 4 Parts for operation and maintenance of MFCs. (From Appendix 1)
35
Electronic equipment
(a) Multimeter. The multimeter is used to measure the electrode
potentials and current.
(b) Resistor. Resistors are connected between the anode and the
cathode.
(c) Electrical wire and alligator clips. These are used for electrical
connections.
Figure 5. Parts for electronic measurements and data acquisition. (From Appendix 1)
Procedure
Students must familiarize themselves with the materials and equipment listed above before
starting the assembly procedure.
Preparing and operating a microbial fuel cell
Assembling a MFC.
a. The following parts are required to assemble a MFC
1. Anode compartment
2. Cathode compartment
3. Two cover sheets
4. Two electrodes (1 graphite, one air electrode)
5. Membrane
6. Four rubber gaskets
7. Twenty-four nuts and 12 bolts
8. Six connectors
9. Two feet of silicon tube
10. Silicon rubber
11. Six clamps
36
b. Diagram that shows the dimensions and relative positions of the MFC parts
6 inch
6 inch
½ inch
1.5 inch
1.5 inch
3 inch
2 inch
1
3 2
¼ inch
0.75 inch ¼ inch
0.75 inch
2.5 inch
5
4
6
1.5 inch thick
5 inch
5 inch
Figure 6 Microbial fuel cell. A) General view. B) Side plates. Port 1: outlet, port 2: air or
nitrogen, port 3: media feed line. C) Growth chamber for anodic or cathodic compartment. D)
Top view of the cell. Port 4 is for the salt bridge for the reference electrode, and ports 5 and 6 are
for the electrical wires connected to the electrodes. E) Electrode configurations used in the
compartments.
Make sure all parts are clean before starting the experiment
b. Cleaning the MFC parts
1. Wash using glass cleaning detergent and tap water
2. Rinse all the parts using tap water
SCE
37
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
Figure 7. Steps for assembling a MFC.
*Be certain to look at the figures to help you with this process
c. Assembling procedure
1. Insert a graphite electrode into the anode compartment, being careful not to
disturb the connections (Figure a)
2. Insert an air electrode into the cathode compartment with the white side facing
out, being careful not to disturb the connections (Figure a)
3. Ensure that the wires are routed through the appropriate ports at the top of the fuel
cell (Figure a, b)
4. Place a rubber gasket on the inner side of the cathode compartment (Figure b)
5. Place the membrane after the gasket placed in step 4 (Figure c)
6. Place a rubber gasket on the other side of the membrane (Figure d)
7. Put the cathode and anode compartments together (Figure e)
8. Place rubber gaskets on the outsides of the anode and cathode compartments
(Figure f)
9. Place the cover plates on the outside of the anode and cathode compartments,
making sure the tube fittings are located in the appropriate location (Figure g, h)
10. Insert the ready-rod (including a washer on each end). Tighten (hand-tight is
enough) using wing nuts. DO NOT overtighten, because you may break the
polycarbonate. If the reactor leaks when filled with liquid, tighten it a ¼ turn
(Figure h,i)
11. Attach silicon tubes (2-inch) to the connectors (Figure j) at the bottom of the
cover plates
12. Close the inlets and outlets using clamp stoppers (Figure j)
13. Apply silicon rubber to close the openings of the compartments
14. Let the silicon rubber cure for 30 minutes.
38
ACTIVITY #5 Loading the fuel cell
(Adapted from Appendix 1)
Purpose
The purpose of this activity is to learn how to prepare growth mediums, inoculate microbes
and load the fuel cells for electrical generation. Once the fuel cells are loaded, students will
measure the cell potential.
Prerequisite Knowledge
Students will need to understand MFC and cellular respiration of microbes. Students will
need to know how to properly use a scale for measuring chemicals, a graduated cylinder,
syringe and a magnetic stirrer. Students will also need to know how to operate a multimeter
and be able to plot data on a graph.
Instructional Strategies
The teacher should assist students in obtaining the chemicals, using the scales and magnetic
stirrers (if available).
Preparing inoculation and growth medium
The MFC growth medium may be altered for different experiments. The nutrients and type
of yeast can be altered depending on which variables you want to test. You need to
determine the volume of your fuel cells and modify the medium amount accordingly.
Standard growth medium for MFC inoculation and energy generation
Growth medium for Saccharomyces bayanus
1L Water
120 g light hopped Malt
1. Weigh the chemicals and put them inside a 1000-ml pyrex bottle. DO NOT mix
the chemicals. After weighing each chemical clean your spatula so you won‘t
contaminate other chemicals.
2. Add 1L of water in the 1000-mL pyrex bottle
3. Mix until all the chemicals are dissolved well. You can do this by swilling the
flask for several minutes.
4. Pour broth in a container that is heat resistant, cover loosely.
5. Bring broth to a boil for 10 minutes (either in a microwave or on the stovetop)
allow to cool to room temperature. Make sure you stir so broth does not burn.
39
6. Divide the medium in half. One half will be used to inoculate the yeast and the
other half will be used to load the fuel cell.
7. Cover the broth that you will use to load the cell with tinfoil. This is your growth
medium for electricity generation.
8. Weigh out .13g of champagne yeast (Saccharomyces bayanus).
9. Add yeast to the remaining half of the cooled broth and swill to distribute yeast.
This is your stock medium.
10. Cover yeast stock medium with tinfoil and allow to sit in a warm location, out of
direct sunlight, for 30 minutes or overnight, whichever works best for your lab.
Cathode compartment
You need to determine the volume of your fuel cells and modify the buffer amount
accordingly.
Fill with a buffer (pH = 7) of the composition shown below:
Table 1. Buffer composition
Components Formula Composition (g/L)
Disodium phosphate Na2HPO4 1.825
Monopotassium phosphate KH2PO4 0.35
1. Weigh each chemical and put them inside a 1000-ml pyrex bottle.
2. Add 1L of water in the 1000-ml pyrex bottle containing the buffer chemicals.
3. Place a magnetic stirrer in the bottle and stir on a stir plate for 15 minutes. Make
sure all chemicals are dissolved before continuing.
Starting the MFC Make sure you have the following items ready:
1. Assembled MFC
2. Growth medium for electricity generation
3. Stock culture containing yeast
4. 20-ml syringe (2)
5. Buffer
6. Air pump and tubing
40
Use the following procedure to start the MFC
(All measurements are assuming the volume of the anode chamber is 100ml)
1. Obtain stock culture and growth medium
2. Remove the silicon rubber from the two inlets of the cathode at the top of the
cathode compartment
3. Inoculate the MFC:
a. Take a 20-mL syringe
b. Open the stock culture vial
c. Put 50ml of the stock culture inside the anode compartment through one of the
tubes. Take care to point the tube down towards the table as broth may expel
out the tube during the filling process.
4. Using the same syringe, fill the anode compartment with 50ml of growth medium
for inoculation
5. Clamp tube after loading fuel cell
6. Immediately shake the MFC a little so that the culture is mixed well
7. Fill the cathode compartment with buffer using the second syringe.
8. Pump air into the cathode at a moderate rate through the bottom tube.
9. Remove the clamp from the upper tube on the anode chamber.
10. Pump air into the anode chamber through the bottom tube. Keep pumping air at
the smallest rate possible. You may need to apply a clamp to keep the airflow to a
minimum. The flow should be one bubble every few seconds. If too much oxygen is
delivered, the electrons will not generate electricity but instead attach to the oxygen.
Operation of the MFC in batch mode
After starting the MFC the following steps are required for the operation of the MFC.
11. Maintain air flow to the anode compartment to reduce the pressure from the CO2
generation.
12. Maintain air flow to the cathode compartment continuously
13. Maintain the fluid levels in the chambers by adding growth medium to the anode
side as needed and buffer to the cathode side as needed. This will ensure that the
entire electrode is available for electron and proton exchange.
Monitoring the potentials of the anode, cathode and MFC and the current of the MFC
We use a multimeter to monitor the electrode potentials and current.
Connecting the MFC to the multimeter
14. Make sure the black wire on the multimeter is plugged into the black receptacle
and the red wire is plugged into the red receptacle on the lower right corner of the
multimeter.
15. Turn the multimeter on to the mV setting
16. Connect the red wire to the cathode
17. Connect the black wire to the anode
18. Record the volts for the anode and cathode, this is your cell potential
19. Cell potentials will be measured every hour for 3 days.
20. Prepare a graph of cell potential versus time in Excel.
41
Questions for MFC
1. Why is it important to handle the electrical components of the fuel cells with care?
2. What is the purpose of the membrane separating the two chambers?
3. Why should you point to filler tube down once you remove the syringe?
4. What is the charge on the anode?
5. What is the purpose of the wires that come out of the top of the fuel cell?
6. Why is it important to maintain the level of the fluids in the chambers?
42
ACTIVITY # 6 Effects of Various Nutrients on Cell Potential
Purpose:
The purpose of this activity is for students to design an experiment to determine if different types
of nutrients affect cell potential in the microbial fuel cell. This activity is unguided inquiry
where the students develop a problem, hypothesis, equipment list, procedure and data table. The
students must then analyze the data and write up a conclusion of their findings.
Prerequisite Knowledge
Students will have learned how to use all materials and equipment in previous labs.
Equipment
See equipment lists for Activities #4 and #5.
Procedure
Part A: Predicting and Planning an Investigation
You are given the task of determining the effects of nutrients on cell potential in a Microbial
Fuel Cell.
1. In your science notebook, state the problem you are trying to solve in this activity.
2. Develop a hypothesis that states what you predict the outcome of the experiment
is and a reason for your prediction.
3. Work as a group to determine which materials you will need for this experiment.
Keep in mind that you will only have 2 microbial fuel cells to use for your experiment.
4. Be sure to include a data table for recording your results.
5. Your problem, hypothesis, materials list, procedure and data table must be
submitted for review and accepted by your teacher before beginning the investigation.
43
Part B: Conducting your investigation
After receiving your teacher‘s approval for your experiment, conduct the investigation.
Conclusion
In your conclusion, Restate the problem you were trying to solve, whether or not the hypothesis
was correct, and evidence from the experiment that supports your decision about the hypothesis.
Explain any problems or issues you encountered while conducting the experiment and why or
why not your hypothesis may have been correct/incorrect.
44
Appendix 1
MICROBIAL FUEL CELLS
EDUCATION MODULE
Washington State University, The School of Chemical Engineering and Bioengineering
Prepared by Alim Dewan and Haluk Beyenal
Washington State University, The School of Chemical Engineering and
Bioengineering
This module was tested for the first time in the 2007 Fall semester ChE 475, Bioprocess Engineering, class. Copyright: Washington State University, The School of Chemical Engineering and Bioengineering, WA 99163, USA.
45
INTRODUCTION This manual will guide you to run a microbial fuel cell (MFC) experiment in the laboratory. In the introduction, you will be introduced to the concepts of microbial fuel cells. “What is a microbial fuel cell?”, “How does it work?” and “What are the applications of microbial fuel cells?” are questions you have in your mind already. In the introduction we will try to find the answers to these questions. Important theories and definitions that are required for analyzing MFC experimental data will also be discussed in this section.
1.2. What is a Microbial Fuel Cell? A microbial fuel cell is an electrochemical device that generates electricity directly from organic chemicals, using microorganisms to catalyze the redox reactions. First of all, it is a “fuel cell,” which is a device that uses electrochemical reactions to produce energy. In modern energy technology, fuel cells replace the conventional high-temperature combustion devices that generate energy from fossil fuels. Secondly, a fuel cell is composed of microorganisms which are used to produce electricity. In nontechnical terms the microorganisms consume organic chemicals to produce energy for their survival, and we collect electrons from the energy-producing pathway to produce electricity. Technically, the microorganisms act as catalysts that accelerate the redox reactions needed for the fuel cell. We can think of a MFC as a battery. The reaction mechanisms are similar. The difference is that in a MFC the fuel can be stored outside the cell and supplied continuously, while in a battery the fuel is limited and stored inside the cell. Electrochemical cells are driven by redox reactions. A redox reaction is a combination of two half reactions: one, the oxidation half reaction, liberates electrons and the other, the reduction half reaction, consumes electrons. In a MFC the half reactions, oxidation and reduction, are separated by a cation exchange membrane. The electrode at which the oxidation reaction occurs is called the anode and the electrode at which the reduction reaction occur is called the cathode. Figure 1 shows a schematic diagram of a MFC and its components.
46
Figure 1. Schematic diagram of MFC operation and components.
The main components of a microbial fuel cell are the anode, the cathode and the proton exchange membrane. In the anode, the microorganisms are grown anaerobically. Fuel, organic matter that can be oxidized anaerobically by microorganisms, is pumped into the anode compartment. The microorganisms oxidize the fuel and derive electrons from the oxidation of the fuel, then transfer them to the solid electrode. Then the electrons are transferred through an external circuit to the cathode. This electron transfer through an external circuit is used to power electronic devices. The protons that are produced during the oxidation of the fuel diffuse from the anode to the cathode through the proton exchange membrane to complete the circuit and balance the charge. An electron acceptor (usually oxygen) is pumped into the cathode compartment, where it accepts electrons from the anode through the external circuit while the protons diffuse through the membrane.
1.3. How does an MFC work? Why did scientists, at the very beginning, think that microbes could be used in fuel cells? The answer to this question may help you to understand the idea behind the MFC. To find the answer let us examine the energy generation process of a single bacterial cell. The energy generation process involves many different reactions, but the overall reaction can be summed up as a redox reaction. In that redox reaction, one chemical is oxidized, liberating electrons, and another chemical is reduced by accepting the electrons. For example, Red 1 (reduced chemical 1) is a carbon source which is oxidized by microorganisms to form an oxidized species, liberating an electron. The reaction can be written as Red 1 = Ox 1 + e-
Eq 1
According to charge conservation, this electron must be accepted by an electron acceptor. As shown in Figure 2A, an electron acceptor, Ox 2 (oxidized chemical 2), is
Electron acceptor
PE
M
Anode
Cathode
Microorganisms Fuel
(Electron donor)
Oxidation products Reduction
products
Load
47
reduced by the electron liberated by oxidation and is converted to Red 2 (reduced chemical 2). Ox 2 + e = Red 2
Eq 2
Thus, the redox reaction comprises the oxidation of fuel and the reduction of an electron acceptor. The overall reaction can be written as Red 1 + Ox 2 = Ox 1+ Red 2
Eq 3
Figure 2. Schematic diagram of how the microbial redox reaction is modified in MFCs. A) The
microbial energy generation process involves a redox reaction. The idea of the MFC lies in
separating the oxidation and reduction environments by membrane or salt bridge and transferring
the electrons through an external circuit to maintain the charge balance. B) A schematic diagram
of how the oxidation and reduction environments can be modified to make MFCs.
The idea behind microbial fuel cells is separating the oxidation and reduction environments in such a way that the electrons can be transferred through an external
e-
Oxidized chemical (Ox 1)
Reduced chemical, an electron donor (Red 1)
Oxidized chemical, an electron acceptor (Ox 2)
Reduced chemical (Red 2)
Cell
Oxidation Reduction
A
B
e
-
Ox 1
Red 1
Cell
Ox 2
Red 2
Anode
e-
Cathode
ATP
48
circuit. The dashed line in Figure 2A is the position where, conceptually, we can separate the two environments. The separation is shown schematically in Figure 2B. The two environments are separated by a membrane (dashed line). The environment where the microorganisms grow and oxidize the fuel is called the anodic compartment, and the environment where the electron acceptor is reduced is called the cathodic compartment. Now the question may arise as to how microbes can produce energy if we do not allow them direct contact with a soluble electron acceptor. The answer is the solid electrode. Microbes can use a solid electrode to transfer the electrons that are liberated in the oxidation process. As depicted in Figure 2B, the electrode placed in the anodic compartment collects the electrons from the microbes and transfers them through the external circuits to the cathode where the electron acceptor (Ox 2) is reduced. For example, if glucose is the reduced chemical that is metabolized by the microorganism, the overall reaction is written as (Please see glucose metabolism discussed later).
ATP38OH6CO6O6OHC 2226126
Eq 4
In this reaction, the oxidation of glucose is written as e24H24CO6OH6OHC 226126
Eq 5
and the reduction of oxygen is written as
OH12e24H24O6 22
Eq 6
If the anode and the cathode are separated by a membrane, we can draw the diagram of the MFC as in Figure . This diagram shows where the reactions occur and what the products in the anodic and cathodic compartments are.
49
Figure 3. Glucose oxidization by a microorganism in a MFC.
How to calculate the number of electrons available to be transferred from the organic compound to the electron acceptor The number of electrons that are available from the oxidation of an organic compound is calculated using the concept of degrees of reduction (Shuler and Kargi, Chapter 7, page 202-205). The degree of reduction of an organic compound (say glucose) is defined as the number of available electrons per gram-atom (similar to the number of gram-moles) of C. The total number of electrons available is calculated by multiplying the degree of reduction by the number of gram-atoms in the compound.
Respiration 24 e
-
6O2
6CO2
C6H12O6 24 H
+
12 H2O
P
roto
n e
xc
han
ge
mem
bra
ne
Microorganism
Anode Cathode
24 e-
50
Example 1
The reduction number of an element is the same as the balance of that element. The reduction number of glucose (C6H12O6) can be calculated as
6
)2(611246 = 4
where the balance of C is 4, that of H is 1 and that of O is -2. The number of gram-atoms of carbon in one mole of glucose is 6. Thus, the total number of electrons available is 6×4 = 24. For lactic acid (C3H12O6), the reduction number is again 4
( 43
)2(31643
), and the number of gram-atoms in the lactate is 3.
Therefore, the total number of electrons available from lactic acid is 4×3 =12. The concept of MFCs can be understood better if we study metabolic pathways. Let us examine the overall processes of aerobic and anaerobic respiration and the fermentation process. Aerobic respiration of Klebsiella pneumoniae Figure 4 is a schematic representation of the aerobic microbial respiration and fermentation of Klebsiella pneumoniae. Remember that Klebsiella pneumoniae is a facultative microorganism, which means it can also respire anaerobically. Here, aerobic respiration is chosen to explain the idea of MFCs. Later, the anaerobic respiration of Shewanella oneidensis (MR-1) will be discussed and compared with the aerobic respiration of Klebsiella pneumoniae.
51
Figure 4. Schematic representation of the overall process of aerobic respiration and fermentation
of Klebsiella pneumoniae. A single cell is mimicked and both processes are shown in the same
figure, although in actuality the processes occur under different conditions. The fermentation
does not require oxygen or the Krebs cycle. The fermentation process uses an organic electron
acceptor. There are MFCs that use fermentation products to produce electricity. For simplicity,
we do not discuss this kind of MFC.
The glycolysis process is common to respiration and fermentation. It produces adenosine 5'-
triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH) while producing pyruvic
acids. In the fermentation process, the pyruvic acids and the electrons carried by the NADH form
fermentation end products (detail is avoided for simplicity). In the respiration process, pyruvic
acid is converted to Acetyle Co-A while producing carbon dioxide and NADH. The Acetyl CoA
enters the TCA or Krebs cycle and produces carbon dioxide, ATP, NADH and FADH2. The
NADH and FADH2 are oxidized, to NAD+ and FAD, in the electron transport chain, and the
electrons which are liberated are transferred through the transport chain by the cyclic oxidation
and reduction of carrier molecules. While the electrons are transferred through the transport
chain, the protons are pumped across the membrane by some carrier molecule, called a proton
pump. A proton concentration gradient forms between the two sides of the cell membrane. Due
Krebs cycle
2 Acetyl CoA
2 NADH
Glycolysis
2 NADH
6 NADH
+2H+
+2H+
+ 6H+
6 FADH2
2 Pyruvic acid
6CO
2
+12H+
10 NADH 6 FADH2
10 NAD+
6 FAD
6 O2
H2O
e-
2 ATP
2 ATP
34 ATP
2CO
2
Fermentation
Fermentation end products
NADH
Electron transport chain
Respiration
Fermentation
H+
Glucose
52
to that gradient, protons diffuse through ATP synthase. When this diffusion occurs, energy is
released and is used by the enzyme to synthesize ATP from ADP and phosphorus.
Anaerobic respiration of Shewanella oneidensis
Figure 5. Anaerobic respiration of Shewanella oneidensis under a fumerate reduction condition.
The pathway for fermentation is not shown here; it is the same as for Klebsiella pneumoniae.
In the anaerobic respiration of Shewanella oneidensis (MR-1) under a fumerate (electron
acceptor) reduction condition, the lactate is converted to pyruvate and then to acetyle-coA. The
Acetyle-CoA enters the TCA or Krebs cycle and produces carbon dioxide, ATP, NADH and
FADH2. In the electron transport chain, the NADH and FADH2 are oxidized to NAD+ and
FAD, respectively, and the electrons are liberated. Then the electrons are transferred through the
transport chain by the cyclic oxidation and reduction of the carrier proteins possessed in the
chain. While the electrons are transferred through the transport chain, the protons are pumped
across the membrane by proton pump. A proton concentration gradient forms between the two
sides of the cell membrane. Because of this gradient, protons are diffused by ATP synthase.
When this diffusion occurs, energy is released and is used by the enzyme to synthesize ATP
from ADP and phosphorus. The electrons transferred through the electron transport chain reduce
the fumerate to form formaldehyde. The whole respiration process can be summarized as
LactatepyruvateacetateCO2. When we use Shewanella oneidensis in our anodic
TCA cycle
2 Acetyl CoA
2 NADH
2 NADH
6 NADH
+2H+
+2H+
+ 6H+
6 FADH2 6CO
2
10 NADH 6 FADH2
10 NAD+
6 FAD
Fumerate
Formaldehyde
e-
2 ATP
2 ATP
34 ATP
2CO
2
Electron transport chain
H+
Pyruvate
Lactatee
Acetate
53
compartment, we don‘t use fumerate because instead of fumerate, we want to transfer electrons
to the solid electrode and generate current.
Electron transport chain
An electron transport chain consists of a sequence of carrier molecules that are capable of
oxidation and reduction. The carrier molecules are flavoproteins, cytochromes and ubiquinones
(Figure 6). The electron transport chain releases energy (ATP) as the electrons are transferred
from higher-energy compounds to lower-energy compounds. Keep in mind that, according to the
modern convention, electrons flow from a lower potential to a higher potential. If there are two
compounds having different redox potentials, the electrons will flow from the compound with
the lower redox potential to the compound with the higher redox potential.
Figure 6. Electron transport chain. The electrons pass along the chain in a gradual and stepwise
fashion through the oxidation and reduction of the flavoproteins (FMN), cytochromes (Cyt) and
ubiquinones (Q). Here oxygen is the final electron acceptor, with a redox potential of + 0.816
VSHE. NADH and FADH2, which are produced in glycolysis and the TCA cycle, have redox
potentials of -0.32 VSHE and -0.22 VSHE , respectively. In a MFC the oxygen is replaced with a
solid electrode which accepts electrons and delivers them to the cathode.
FMN
Form
al R
eductio
n p
ote
ntia
l
Cyt b
Q
Cyt
a3
Cyt
a
Cyt
c1
Cyt c
2e + NAD++H+ = NADH -0.32 VSHE
2e +FAD++2H+=FADH2 -0.22 VSHE
reduction
oxidation
Electrons flow from lower to higher potential 4H++O2+4e=2H2O
Electron transfer direction
+0.812 VSHE
54
Remember that in a fuel cell the bacteria produce energy by anaerobic respiration. The mechanism of anaerobic respiration is the same as that of aerobic respiration, but the electron acceptor is different. In aerobic respiration, the final electron acceptor is oxygen, whereas in anaerobic respiration the final electron acceptor is a chemical other than oxygen. Some anaerobes (bacteria that can survive only in an anaerobic condition) and facultative anaerobes (bacteria that can survive in both aerobic and anaerobic conditions) can use nitrate, sulfate, carbonate and metals. A solid electrode which can accept electrons can be used for respiration, as happens in MFCs. How are electrons transferred from microorganisms to a solid electrode? So far we know MFCs can produce electricity using electrons released by microbial respiration through the production of NADH and FADH2, which are oxidized in the electron transport chain. The electron transport chain carries the electrons to the final electron acceptor, which in the case of a MFC is a solid electrode. Now the question you may ask is “how are the electrons transferred from the electron transport chain to the electrode?” This question has not been answered yet. However, there are some hypotheses on electron transfer from the electron transport chain to the solid electrode. Keep in mind that researchers are still looking for experimental proof of the hypotheses. So come up with your own explanation that may be a breakthrough in MFC research. Figure 7 summarizes the hypotheses of the electron transport mechanism.
Figure 7. Summary of hypothesized electron transfer mechanisms. A) Redox potentials inside
the cell (Ecell), at the cell wall (EcellW), in the medium (Emedium) and at the anode (Eanode). In an
electrochemical system, the electrons move from lower redox potential to higher redox potential
(see arrow in the figure). In this case, since the electrons are transferred from the cell to the
anode, the potential inside the cell is the lowest and the anode potential is the highest. B) The
electrons are transferred by a mediator. C) The electrons are transferred directly. There is
evidence that certain microorganisms produce nanowires which transfer electrons directly to the
ANODE
Ecell
Emedium
EcellW
wall
Eanode
Cell
B. Mediated electron transfer
C. Direct electron transfer
A. Redox potential in different region
e-
e-
e-
e-
Mediator (M) Cell
Incre
ased re
dox p
ote
ntia
l
55
electrode. Some researchers also believe that cytochrome protein in the electron transport chain
may transfer electrons directly to the anode.
We categorize electron transfer mechanisms into two groups: 1) mediated electron transfer and 2) direct electron transfer.
1) Mediated electron transfer The mediators are redox species that can accept electrons from the electron transport chain and transfer them to the solid electrode. They are also called electron shuttles. The reduced metabolic products (for example hydrogen), and organic (for example, 2-hydroxy-naphthoquinone (HNQ) and organometallic compounds (for example iron-ethylenediamine-tetraacetic-acid (Fe-EDTA)) can be used as mediators. The mediators can also be produced by the cells; these are called microbially produced mediators. For example, the Shewanella species can produce an iron compound that acts as a mediator. The mediators can also be added externally, for example 2-hydroxy-naphthoquinone (HNQ), which is considered an artificial mediator. The mechanism of electron transfer is the same for artificial and microbially produced mediators.
2) Direct electron transfer The cytochrome proteins in the electron transport chain may come in direct contact with the solid electrode and transfer the electrons directly. In addition, the recent discovery of bacterial nanowires leads some researchers to conclude that nanowires are used to transfer electrons directly. The researchers are still working to prove this kind of electron transfer mechanism experimentally. So, while running the experiments, keep your eyes open: you may discover something new about the electron transfer mechanism.
1.3 Concepts Electric potential Absolute electric potential can not be defined for a single point in space: it must be determined
with respect to the potential of another point in space. The electric potential at location A with
respect to location B is defined as the work needed to bring one unit of positive electric charge
from location B to location A. For example , an electric charge qo is transferred from A to B.
Location A has electric potential energy EPEA, location B has electric potential energy EPEB, and
the difference in electric potential energy between these locations is EPEB – EPEA = EPEAB. The
numerical values of EPEA and of EPEB are not known. Only the difference between them,
EPEAB, is known: it is equal to the work needed to move the test electric charge from A to B. To
make the result of the computation independent of the magnitude of the test electric charge, the
energy change is computed per unit of the electric charge transferred.
56
o
AB
o
AB
o
AB
q
EPE
q
)EPE -(EPE
q
W
Eq 7
The difference in electric potential energy of the unit positive electric charge between A and B is
referred to as the electric potential difference between these two locations:
VVq
EPEAB
o
AB
Eq 8
In an electrochemical system, the electric potential of an electrochemical cell is the work needed
to move one unit of positive charge from a standard hydrogen electrode to the indicator electrode
in the cell. The potential of the hydrogen electrode is zero by convention.
For example, using SI units, the work needed to take one unit charge from a hydrogen electrode
to electrode ‗x‘, the potential of electrode ‗x,‘ is
V1C1
J1
Eq 9
Electrode and cell potentials Redox reactions are composed of two half-reactions: one substance donates the electrons and the
other substance accepts the electrons. The electrons have the tendency to move from one
substance to the other because the substance that donates the electrons has a lower affinity for the
electrons than the substance that accepts them. As a result of the electron transfer, the substance
that donated the electrons is oxidized and the substance that accepted the electrons is reduced.
The affinity of a substance for electrons can be evaluated and cataloged for the standard
conditions as the potential of the half-reaction in which this substance participates: the higher the
potential of the half-reaction, the higher the affinity of the substance participating in that half-
reaction for the electrons. As a result, substances can be compared by evaluating their affinity for
electrons, and it can be predicted which substance will donate the electrons and which substance
will accept the electrons, which substance will be oxidized and which will be reduced. Whether
the actual transfer of the electrons occurs depends on kinetic limitations, and the kinetics of these
reactions are determined experimentally. Technically, the difference between the potentials of
the half reactions is called the cell potential, and it is proportional to the Gibbs free energy
change resulting from transferring the electrons from one half-reaction to the other. In chemistry
the tendency of a reaction to occur is quantified by computing the Gibbs free energy change. In
electrochemistry this same tendency is quantified by computing the cell potential. The two
computations are equivalent: the cell potential is the Gibbs free energy change expressed in
electrical terms, and the Faraday constant is used as the conversion factor.
57
The Faraday constant (F = 96,485 C/mol) is equal to the electric charge of one mole of electrons,
and it appears in many electrochemical equations. To compute the Faraday constant the electrical
charge of a single electron, which is equal to 1.602×10-19
coulombs, is multiplied by the number
of electrons in one mole of electrons, which is equal to Avogadro‘s number, 6.0238×1023
: the
product of these numbers is the Faraday constant, 96,495 coulombs per mole of electrons.
The common expressions that use the Faraday constant are: (1) the Faraday constant multiplied
by the number of moles of electrons (n) transferred between locations, nF, which is equal to the
charge transferred between these locations, and (2) the electric charge transferred between
locations multiplied by the potential difference between these locations expressed in volts, nFE,
which is equal to the energy change. Actually, the latter expression should use ∆E instead of E,
but it is customary to use E because potentials are always measured with respect to another
potential, so E is always ∆E.
The electric charge transferred in a redox reaction, nF, and the energy change, nFE, are often
used in electrochemical computations. From these relations, the Gibbs free energy change in an
electrochemical reaction can be expressed as the equivalent of the potential difference:
nFEG Eq 10
where ∆G is the Gibbs free energy change associated with the electron transfer. The accepted
sign convention is consistent with the convention used in chemical thermodynamics: the energy
of the reactants is subtracted from the energy of the products.
Since the Gibbs free energy change for a simple electrochemical reaction can be computed from
thermodynamics, and the free energy change in a redox reaction is equivalent to the potential
difference, it is also possible to compute the equivalent potentials for these reactions. Such
computations yield ―half-cell potentials,‖ which have exactly the same use as the computations
of Gibbs free energy change in these reactions, but the half-cell potentials are expressed in units
used in electrochemistry, volts. Computing cell potentials from Gibbs free energy changes does
not add any additional information: the cell potential is just the Gibbs free energy change divided
by the electric charge transferred in that reaction (nF). The sign convention is such that reactions
are spontaneous when their cell potentials are positive.
nF
GE
Eq 11
This sign convention is consistent with the discussion earlier: electrons tend to move from
locations with lower potentials to locations with higher potentials. If we use the thermodynamic
convention and subtract the potential of the final destination (the higher potential) from the
potential of the initial location (the lower potential) the sign of the resulting change is positive,
58
which indicates that the transfer of electrons between these locations is spontaneous. A negative
change of Gibbs free energy is equivalent to a positive potential.
Because the number of electrons transferred, n, and the Faraday constant, F, are constant for a
given reaction, the cell potential, E, is just another way of expressing the Gibbs free energy
change for that reaction, ∆G. For the redox reaction described by the following stoichiometry:
r(reactant) + ne = p(product)
Eq 12
r
p
o
ttanreac
productlnRTGG
Eq 13
where r and p stand for the stoichiometric coefficients associated with each of the reactants and
products,introducing appropriate expressions for the relations quantifying the Gibbs free energy
change yields:
oo nFEG
nFEG
This can be written as:
r
p
o
ttanreac
productln
nF
RTEE
Eq 14
This equation is called the Nernst equation. At 25oC (T = 298oK), assuming F = 96,485 C/mol, R=8.314 J/mol.K,
r
p
o
ttanreac
productln
n
059.0EE
Eq 15
We measure the electrode potential as the difference between the electrical potential of an electrode and the electrical potential of a reference electrode.
Potential described against reference electrodes The potentials of half-cells can be calculated with respect to reference electrodes other than SHE as distances between respective rungs on the potential ladder (Figure 8). For example, if an arbitrary half-reaction A has a half-cell potential with respect to SHE
59
equal to +0.362 V, then it has a half-cell potential of 0.362 V – 0.197 V = 0.165 V with respect to the saturated Ag/AgCl electrode. Similarly, if the half-cell potential of an arbitrary half-reaction B is –0.185 V with respect to SHE then its half-cell potential with respect to SCE is 0.241 V + 0.185 V = 0.426 V. As for the signs of the computed potentials: if the half-cell potential of the arbitrary half-reaction is above the potential of the selected reference electrode, the sign of the potential is positive, and vice versa. Consequently, the computed half-cell potential for half reaction A is +0.165 V, while the half-cell potential for half reaction B is -0.426 V. If the half-cell potential of an arbitrary half-reaction is between the potentials of SHE and, say, SCE, then it is positive with respect to SHE and negative with respect to SCE.
0V 2H+ + 2e H2(g) SHE
+0.241V Hg2Cl
2(s) + 2e 2Hg(l) + 2Cl- SCE
+ 0.197V AgCl +e Ag(s) + Cl- Saturated Ag/AgCl
+ 0.362V Half-cell A
- 0.185V Half-cell B
POTENTIAL (+)
Figure 8. Relative positions of various reference electrodes.
MFC potential with mediated electron transfer Let us consider a MFC in which the electron is transferred from the microorganism to the electrode by a mediator (M). 1) Calculation of anode potential. The mediator (Mred) that is reduced by accepting electrons from the microbial respiration system is oxidized at the anode according to the following reaction:
Eq 16
Mred is the reduced form of the mediator and Mox is the oxidized form of the mediator. The anode potential (EA) is calculated using the Nernst equation:
eMM oxred
60
Eq 17
where (M)ox and (M)red are the activities of the oxidized and reduced forms of the
mediator, typically substituted for by the molar concentrations. o
ME is the standard redox
potential of the mediator. The challenge we have in this calculation is knowledge of the mediator. Generally we do not know the mediator; in this case we make an approximation and use artificial redox mediators for the calculation. 2) Calculation of cathode potential. If oxygen is reduced the cathode according to the following reaction
OH2e4H4O 22
Eq 18
the cathode potential is
4
o
o
OC
H.p
1log
4
059.0EE
2
2
Eq 19
3) Calculation of the cell potential. The cell potential of the MFC is determined by subtracting the anode potential from the cathode potential according to the following equation:
ACCell EEE
Eq 20
This cell potential is called the open circuit potential (OCP) because no load is applied to the MFC (there is no current flow from anode to cathode) and the electrode reactions are in equilibrium. Note: E is used to denote the equilibrium potential and V is used for the actual potential (when there is a load or when a current flows from the anode to the cathode).
Overpotentials and actual cell potential. The potentials of an electrode at equilibrium and when a load is applied are different. An overpotential is defined as the difference between the equilibrium potential and the potential when a load is applied. Think about yourself as a student: you have homework with a deadline one month later. Generally you do not work on this homework although you have the energy (potential) to do. However, if the deadline is the next day, you start working on the homework and work
red
oxo
MA)M(
)M(log
2
059.0EE
61
till late at night; at the end you become tired (your potential becomes low). Your equilibrium potential is your energy at the beginning when you are just starting to do your homework. Your overpotential is the difference between your potential at the beginning and your potential at the end: this is the driving force that makes your homework get done. Similarly, the overpotential in a MFC makes current flow. Because of losses of the equilibrium electrode potential or the overpotential, the actual cell potential of a MFC is always less than the open circuit potential. Here the word “loss” is used to show a negative sense, because when we apply a load to a MFC the electrode potential changes in a direction that is undesirable. The potential losses are actually called kinetic loss or overpotential. There are three types of potential losses or overpotentials: 1) activation overpotential, 2) ohmic overpotential and 3) concentration overpotential. Figure 9. is a theoretical plot of the change of an electrode potential with increase in current. The activation overpotential (Region I) is due to the activation energy needed for the oxidation/reduction reaction which occurs during the electron transfer from the compound to the solid electrode or from the electrode to the compound. The ohmic overpotential (Region II) includes the loss due to resistivity of the bulk electrolyte. The concentration overpotential (Region III) is due to the change in concentration of the electrolytes at the surface of the electrode during current flow; this is also referred to as diffusion loss.
Figure 9. Change of electrode potential with current. Region I: Activation overpotental, Region
II: ohmic overpotential and Region III: concentration overpotential.
In an MFC, both electrodes may have all the losses described above. But remember, the directions of the potential changes of the anode and the cathode are opposite. When a load is applied to a MFC the anode potential changes towards the cathode and the cathode potential changes towards the anode. These changes can be depicted as shown in Figure 10. The cell potential (Vcell) is the difference between the equilibrium potential and the total overpotentials.
Current (mA)
Potential (V)
EA
I
II
III
62
Figure 10. Change of anode and cathode potentials when a load is applied. EA and EC are the
equilibrium potentials of the anode and the cathode, respectively. Vcell is the cell potential at
current i.
The actual cell potential can be written as
Eq 21
or
ACcell VVV
Eq 22
where Vcell is the actual cell potential, EC is the cathode potential at equilibrium, EA is the
anode potential at equilibrium, CI , is the activation overpotential at the cathode, AI , is
the activation overpotential at the anode, CIII , is the concentration overpotential at the
cathode, AIII , is the concentration overpotential at the anode, and CII , is the ohmic
overpotential for both the anode and the cathode. Now, if we make one electrode, say the cathode, very large compared to the other electrode, say the anode, the potential of the cathode may not be affected or the change will be very small (blue dashed line for cathode). In that case we can ignore the overpotential of the cathode. Then, the cell potential will be
EA
EC
Anode overpotentials
Cathode overpotentials
Vcell
Cathode
Anode
Current i
+
VA
VC
Po
ten
tial
)E()E(V A,IIA,IIIA,IAC,IIC,IIIC,ICcell
63
)E()E(V A,IIIA,IIA,IACcell
Eq 23
or
ACcell VEV
Eq 24
Keep in mind that we also assumed that the cathodic current density is significantly higher than the anodic current density. Example 2
If the anode potential of a MFC is changed as shown in Figure 11 and the cathode potential, EC, remains constant at 0.294 V, what are the overpotentials of the anode and the cathode at 1.5 mA? The equilibrium anode potential is -0.55 V. What is the cell potential at that current?
R2 = 0.9982
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0 0.5 1 1.5 2
Current (mA)
An
od
e p
ote
nti
al (V
)
Figure 11. An experimental MFC result. At equilibrium the anode potential was 0.55 V. The
external resistance between the anode and the cathode varied from 10 kΩ to 0.5 kΩ. The cathode
potential was constant for the range of load applied.
Answer: Open circuit potential, Ecell = EC-EA= 0.294-(-0.55) = 0.844 V Overpotentials: For this MFC experiment in which the cathode potential was constant:
CI , = CII , = CIII , = 0. From Figure 11, AI , =0.05 V, AII , =0.04 V, AIII , = 0.194 V and
EA= -0.55 V. Cell potential: From equation 22, Vcell = 0.294 - (-0.55 + 0.05 + 0.04 + 0.194) = 0.56 V
64
How to calculate current: electrode kinetics and the Butler-Volmer equation When the current flows, and electrical charges are transferred between the electrodes and the
dissolved species in the solution, the electrode acts as a chemical reactant and is subjected to the
same rules of chemical kinetics as any other reactant. However, the existence of an electrical
field in proximity to the surface of the electrode introduces additional factors that need to be
taken into consideration in quantifying the kinetics of the reaction. The potential difference
between the electrode and the solution generates an electric field. The electroactive species, the
reactants in the redox reactions, are subjected to this electric field and behave differently than
they behave in the absence of the field. Some electrically charged particles will find it easier to
approach the surface of an electrode because they are electrostatically attracted to the surface,
and other electroactive species in the solution will find it difficult to approach the surface of the
electrode because they are repulsed by the electrical field.
In a reversible electrode reaction process at equilibrium, the current does not
flow in the external circuit, and the fluxes of the electrical charges across the interface are equal
in the two directions. The current measured in the external circuit is the result of the net
difference between the flux of electric charges across the interface in one direction and that in the
other direction. For the oxidation reaction of the mediator
Eq 25
the current can be measured using the Butler-Volmer equation, shown in equation 22.
Eq 26
where, i is the net current, io is the exchange current, is the overpotential, F is the
Faraday constant, R is the universal gas constant and T is the temperature in Kelvin,
c and a are the transfer coefficients, and 1ac . In most cases it is assumed
that c = a =0.5, which indicates that the activation energy barriers of the oxidation and
reduction reactions are the same. The overpotential (η) is expressed as
eqEE
Eq 27
RT
Fexp
RT
Fexpii ac
0
eMM oxred
65
where E is the potential difference between the electrode and the solution when the
electrode is not in equilibrium and eqE is the potential difference between the electrode
and the solution at equilibrium. If the electrode is at equilibrium, η = 0, then the rate of the anodic reaction is equal to the rate of
the cathodic reaction, and the anodic current density equals the cathodic current density. This
special case of current density at equilibrium is called the exchange current density, io. It cannot
be measured in the external circuit: at equilibrium the current in the external circuit is equal to
zero.
Example 3
Compute the anodic, cathodic, and net current densities in the following system:
αa = αc = 0.5
io = 1 mA/cm2
Surface area of the electrode, A = 1 cm2
Use (1) cathodic polarization η = - 0.1 V and (2) anodic polarization η = + 0.1 V.
To simplify computations note that F/RT = 38.95 coulombs × J-1
Current The current of a MFC is measured experimentally using an electrometer or calculated using the following equation when a load (R in ohms) and a potential drop (Vcell in volts) across the load are measured using an electrometer.
ext
cell
R
VI
Eq 28
Example 3: Using equation 27, if the cell potential is Vcell = 0.674 V and load = 600 Ω, the current is I = 0.674/500 = 0.00112 A = 1.12 mA.
66
Power and power density The performance of a MFC is evaluated by calculating power generation. Power generation is calculated using the following equation:
ext
2
cellcell
R
)V(I.VP
Eq 29
Figure 12. A load is applied between the anode and the cathode. ‗Am‘ is the ammeter connected
in series and the ‗Vm‘ is the voltmeter connected parallel with the load. If we know the load, Rex,
and the Vcell, we can calculate the power by calculating the current using equation 27. In that
case we do not need to measure the current using an ammeter.
where P is the power (in watts); Vcell is the potential drop across the load (in volts), which is equal to the cell potential; I is the current flow; and Rext is the external resistor applied between the anode and the cathode. Figure 12 shows how to connect the ammeter and voltmeter to the MFC to measure the potential and the current.
Power density, e
aA
PP , where Ae is the geometric surface area of the electrode.
Example 4
If cell potential Vcell = 0.674 V, load R= 600 Ω, and electrode surface area Ae = 27 cm2. Calculate power density (Pa). I = 0.674/500 = 0.00112 A = 1.12 mA, Power = Vcell × I = 0.674 × 0.00112 = 0.00075 watts, and Power density Pa= 0.00075/ 27 = 2.8 × 10-5 watt/cm2.
Cathode
Me
mb
rane
Anode
Load (Rext) (Rex)
Am
Vm
67
Faradic efficiency
Faradic efficiency ( c ) is defined as the total charge produced from a substrate divided
by the maximum possible charge production from the same substrate.
chargeavailablellyTheoretica
productionchargeActual
M
SnF
dtI
t
0
c
The total charge actually produced is calculated integrating the current over time. The maximum possible charge production is calculated multiplying the number of moles of
substrate reacted (M
S) with the faraday constant (F) , where ΔS is the amount of
substrate consumed during time 0 to t, M is the molecular weight of the substrate, and n is the number of moles of electrons involved in the redox reaction per mole of
substrate. c is the faradic efficiency. This is also called “coulumbic efficiency.”
Example 5
When a MFC using a mixed culture of bacteria was fed continuously with a medium of 1 g/L glucose and a load of 337 Ω was applied, the current generation was as shown in Figure 15 . The feed rate was 0.05 L/day and the experiment was run for 48,000 sec. What was the coulombic efficiency if the glucose concentration in the effluent was 0.4 g/L?
0.0000
0.0020
0.0040
0.0060
0.0080
0.0100
0 8000 16000 24000 32000 40000 48000
Time (sec)
Cu
rren
t (A
)
Figure 13. Current vs. time profile when a 330-Ω resistor was applied.
t
dtI0
. = actual charge production = area under the curve shown in Figure 13 = 239.7
coulombs. the area under the curve is calculated by a numerical method using MS Excel.
68
F = Faraday constant = 96500 coulombs/mole of electrons n= 24 for glucose oxidation M= 180 g/mole of glucose
S = amount of substrate consumed = (1-0.4)g/L × 0.1 L/day × 48000/86400 day =
0.033 g
and M
SnF
= 96500 × 24 × 0.033/180 = 424.6 coulombs
Thus, coulombic efficiency
M
S.n.F
dt.I
t
0c
= 239.7/424.6 = 56.5 %
Energy efficiency of the MFC Energy efficiency is defined as the ratio of the total energy that can actually be produced to the total energy that could be produced if the substrate were combusted. The heat of combustion is used as the denominator so that the efficiency of the MFC is comparable to the efficiency of energy generation by the thermal process.
energythermalavailablelTheoretica
productionenergyTotal
m.H
dtRI
m.H
dtI.V
inc
t
0
ext
2
inc
t
0
cell
E
Eq 30
where E is the energy efficiency of a MFC, Vcell is the cell voltage, and I is the current
flow. Thus, the integration of Vcell.I over time 0 to t gives the total energy production. The
term inc m.H is the total energy that could be produced by combusting the same
substrate. cH is the heat of combustion (j/mole), and min is the total substrate used
during time 0 to t.
69
Example 6
In the experiment described in Example 4, what is the energy efficiency?
0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.0300
0 8000 16000 24000 32000 40000 48000
Time (sec)
I2R
(watt
)
Figure 14. I2R vs. time. The experimental conditions are the same as in Example 5. Answer: Since the cell voltage profile is not given, we calculate energy efficiency using
inc
t
0
2
Em.H
dtRI
From the I vs. t data we can plot I2R vs. time as shown in Figure 14.
t
0
2 dtRI = 529.7 joules
The heat of combustion of glucose at normal temperature and pressure is cH = 2830
A MFC is sustainable if its electricity generation does not change over time. Since the purpose of
running an MFC is to generate power for a long period continuously it is important that the MFC
be sustainable; otherwise, application of the MFC would be difficult. For a sustainability test, a
constant load is applied between the anode and the cathode and the current or the potential across
the load is observed over time. If the MFC is sustainable, the current drops slowly and ultimately
70
reaches a steady state value, as shown in Figure 15 for 337 Ω. If the MFC is not sustainable the
current decreases over time and does not reach a steady state value (Figure 15, for 10 Ω). Note
that for these experiments the MFC was operated continuously.
Figure 15 also gives us another important message: If we consume natural energy at a rate faster
than that it can be renewed (for example, by using a 10-Ω resistor) we will run out of energy
very soon. However, if we consume the energy at a sustainable rate (the renewal of the energy is
equal to the consumption rate) we can have energy for an unlimited time.
Figure 15. This MFC is sustainable at a low applied load (high resistor, 337 Ω) but not
sustainable at a high load (low resistor, 10 Ω).
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
0 3 6 9 12
Time (hrs)
Cu
rren
t (m
A)
10Ω
337Ω
71
2. EQUIPMENT USED TO OPERATE MFCs and PERFORM MEASUREMENTS This section introduces the equipment and tools required for running MFC experiments. A brief description and some tips for using the tools are given in the figure captions.
Figure 16. Flow diagram of a MFC system. 2.1 Components of a MFC
(a) Anode and cathode compartments. The holes at the top are used to insert electrodes or to make electrical connections to electrodes in addition to inserting a reference electrode.
(b) Anode and cathode cover plates. The anode and the cathode
compartments and the cover plates are fabricated from polycarbonate.
The working volume of each chamber is approximately 250 mL.
1-Anode 2-Cathode 3-Reference electrode 4-Resistor 5-Proton exchange membrane 6-Pump 7-Anode outlet 8-Cathode outlet 9-Control valve 10-Flow meter 11-Flow breaker 12-Growth medium tank 13-Air pump 14-Waste tank 15-Nitrogen gas tank 16-Computer 17-Data logger
0.673V
101
14
15
MFC compartments
16
1 2 3
4
10 10
6
7
8
5
13
11
11
12
17
9
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(c) Cation exchange membrane (C-7000). The anode and the
cathode are separated by this cation exchange membrane.
(d) Graphite electrode. Graphite is used due to its inert structure.
We use graphite as both anode and cathode.
(e) Air electrode. An air cathode composed of Pt wire mesh and
coated with carbon powder is used. The choice of cathode material
depends on the oxidizing agent used for the cathodic reaction. When
oxygen is used as an electron acceptor, carbon materials are used with
Pt or Ni catalysts because plain carbon gives a high kinetic limitation.
(f) Rubber gasket. A minimum of four gaskets are required for
sealing one MFC.
(g) Bolts and wing nuts. Ensure that the structural hardware used to
assemble the MFC is 316L stainless steel. We use 316L stainless
steel due to its resistance to corrosion. During the experiments these
parts will be wet. If we use a lower grade of stainless steel it corrodes
easily.
(h) Silicon adhesives. Silicon adhesives are used to seal any unused
ports on the MFC.
(i) Conductive epoxy. Conductive epoxy is used to secure electrical
connections between wires and electrodes.
(j) Fittings. The fittings are used to connect silicon tubes to the inlet
and outlet.
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(k) Barbed tube connectors. These are used to connect two silicon
tubes.
(l) Saturated calomel electrode (SCE). We use this as a reference
electrode.
Figure 17. List of parts used to assemble a MFC
2.2 Equipment for operation and maintenance
(a) Silicon tube. Various lengths of tubing are used for the feed and waste
streams.
(b) Flow breaker. This is used to prevent reverse contamination of the
sterile growth medium.
(c) Clamps. These are used to close ports of inlets and outlets of the MFC.
(d) Filter (0.2 μm). Filters are used on the nutrient feed vessel in order to
prevent contamination of the sterile growth medium.
(e) Peristaltic pump and pump controller. The pump is used to regulate
the flow of sterile growth medium into the MFC.
74
(f) Syringe and needle. The syringe is used to inject water into the MFC
prior to autoclaving. It is also used to inoculate the MFC with bacteria and
feed the sterile growth medium into the MFC.
(g) N2 gas cylinder and regulator. The nitrogen is pumped into the anode
chamber to assure an anaerobic environment.
(h) Carboy. A carboy is used to collect MFC waste.
(i) Flask with sterile growth medium. A growth medium suitable for the
given microorganism is prepared. It is essential to properly mix and sterilize
the medium for optimum growth.
(j) Flask with inoculated bacteria. The bacteria are nourished before the
MFCs are started in order to decrease the lag phase.
Figure 18. Parts for operation and maintenance of MFCs.
2.3 Electronic equipment
(a) Multimeter. The multimeter is used to measure the electrode
potentials and current.
(b) Resistor. Resistors are connected between the anode and the
cathode.
75
(c) Electrical wire and alligator clips. These are used for electrical
connections.
(d) Resistor box. A resistor box is used to vary the external resistors
used between the cathode and the anode.
(e) Data logger. We use a HP data logger which can record potentials
and/or current values at preselected time intervals.
(f) Potentiostat. This device measures and controls the potentials of the
electrodes in an electrochemical cell. We can run many electrochemical
experiments using the potentiostat, including polarization,
voltammetry, corrosion measurement, and electrochemical impedance
spectroscopy. The device consists of an electric circuit which controls
the potential across the cell by sensing changes in its resistance and
varies the current accordingly.
Figure 19. Parts for electronic measurements and data acquisition.
3. PREPARING AND OPERATING A MICROBIAL FUEL CELL
3.1. Assembling a MFC.
a. The following parts are required to assemble a MFC 1. Anode compartment 2. Cathode compartment 3. Two cover sheets 4. Two graphite electrodes 5. Membrane 6. Four rubber gaskets 7. Twenty-four nuts and 12 bolts 8. Six connectors 9. Two feet of silicon tube 10. Silicon rubber 11. Five clamps
76
b. Diagram that shows the dimensions and relative positions of the MFC parts
6 inch
6 inch
½ inch
1.5 inch
1.5 inch
3 inch
2 inch
1
3 2
¼ inch
0.75 inch ¼ inch
0.75 inch
2.5 inch
5
4
6
1.5 inch thick
5 inch
5 inch
Figure 20. Microbial fuel cell. A) General view. B) Side plates. Port 1: outlet, port 2: air or
nitrogen, port 3: media feed line. C) Growth chamber for anodic or cathodic compartment. D)
Top view of the cell. Port 4 is for the salt bridge for the reference electrode, and ports 5 and 6 are
for the electrical wires connected to the electrodes. E) Electrode configurations used in the
compartments.
c. Connecting electrical wires to the electrodes To make sure of electrical connectivity we use two-point connections. In a two-point connection, two electrical wires are connected in each electrode at a distance of 2 cm. For the graphite plate electrode, the two holes are made 2 cm apart. The wire is inserted into the hole and filled with conductive epoxy. We use commercially available silver conductive epoxy (www.circuitspecialists.com) to connect the electrical wires with the electrodes. This conductive epoxy offers high electrical conductivity and strong conductive bonding. It is also good for long-term experiments in wet conditions. If we use two-point connections, we can check the connectivity of the wire with the electrodes while running the MFC; in addition, if one connection fails the other may work. The electrical connections are checked
SCE
77
by measuring the resistance between the wires. We consider a resistance of less than 1 ohm between two wires acceptable for a good connection.
d. Cleaning the MFC parts 1. Wash using glass cleaning detergent and tap water 2. Rinse all the parts using tap water
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
Figure 21. Steps for assembling a MFC.
e. Assembling procedure
1. Insert a graphite electrode into the anode compartment (Figurea) 2. Insert a graphite electrode into the cathode compartment (Figurea) 3. Ensure that the wires are routed through the appropriate ports (Figure a,
b ) 4. Place a rubber gasket on the inner side of the cathode compartment
(Figureb) 5. Place the membrane after the gasket placed in step 4 (Figurec) 6. Place a rubber gasket on the other side of the membrane (Figured) 7. Put the cathode and anode compartments together (Figuree) 8. Place rubber gaskets on the outsides of the anode and cathode
compartments (Figuref) 9. Place the cover plates on the outside of the anode and cathode
compartments (Figureg) 10. Insert the ready-rod (including a washer on each end). Tighten (hand-tight
is enough) using wing nuts. DO NOT overtighten, because you may break
78
the polycarbonate. If the reactor leaks when filled with liquid, tighten it a ¼ turn (Figureh,i)
11. Attach silicon tubes (2-inch) to the connectors (Figurej) at the bottom of the cover plates
12. Connect a flow breaker to a silicon tube (6-inch) and connect the tube to the connector at the top of the anodic compartment‟s cover plate
13. Close the inlets and outlets using clamp stoppers (Figurej) 14. Apply silicon rubber to close the openings of the compartments 15. Let the silicon rubber cure overnight.
3.2. Sterilizing the MFC
1. Open two inlets in each compartment
Figure 22. Filling the MFC with deionized water.
2. Fill both compartments with deionized (DI) water. Use a syringe (20-ml) to
pour water inside. Keep a record of the liquid volume of the MFC.
Figure 23. Clamps are used to close the inlets.
3. Close the inlets. Use the clamps shown in Figure 23.
79
Figure 24. MFC placed in an atuoclavable tray.
4. Put the water-filled MFC in a tray (Figure 24). Be sure that the tray is
autoclavable. Otherwise you may ruin the entire setup. 5. Put autoclave tape on it. After autoclaving the color of the tape is changed.
The color change indicates that autoclaving is done. 6. Loosen (slightly) one clamp in each compartment (the clamp that is used
to close the top inlet in each compartment). This step is important: otherwise the inside pressure may damage the silicon seal. When the autoclaving is done, close the feed lines keeping the tray inside the autoclave machine.
Figure 25. The tray with MFC placed inside the autoclave machine
7. Autoclave at 121oC for 15 mins. (We autoclave the MFC to kill all the
bacteria present in the MFC, because later we will grow a single specific type of bacteria and we do not want any other bacteria to grow in the MFC.)
8. When autoclaving is done, close the inlets that were loosened to release the pressure. Take the tray out.
9. Cool the MFC down to atmospheric temperature. 10. Take the MFC out of the autoclave and cool it down to room temperature.
3.3. Growth medium composition
In this experiment we use two different mediums. The first medium is used to prepare the inoculum. The second medium is used to run the MFC for electricity generation. For inoculation we use LB broth (LB is an abbreviation for Lysogeny broth, a nutritionally rich medium. It is also known as Luria broth or Luria-Bertani broth.) The compositions of both types of growth medium for two different bacteria are shown below:
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a. Growth medium composition for Shewanella oneidensis (MR-1)
Table 2. Growth medium for Shewanella oneidensis (MR-1)
For inoculation
For MFC
Chemical Formula Composition (g/L)
Composition (g/L)
Tryptone - 10 -
Sodium chloride NaCl 5 -
Yeast extract - 5 1
Na-Lactate C3H5O3-Na - 11.23
Potassium phosphate
KH2PO4 - 0.77
Disodium phosphate Na2HPO4 - 0.47
Ammonium chloride NH4Cl - 1.5
Potassium chloride KCl - 0.1
b. Growth medium composition for Klebsiella pneumoniae
Table 3. Growth medium for Klebsiella pneumoniae
For inoculation
For MFC
Name Formula Composition (g/L)
Composition (g/L)
Tryptone - 10 10
Sodium chloride NaCl 5 5
Yeast extract - 5 5
Disodium phosphate Na2HPO4 - 1.825
Potassium phosphate KH2PO4 - 0.35
Glucose C6H12O6 - 1
3.4. Preparing and sterilizing the growth medium
a. Growth medium for MFC inoculation If there is ready growth medium for inoculation you can use it. However, you may need to go through the process with your TA.
1. Put about 400 ml of DI water in a 1000-mL pyrex bottle 2. Put the bottle on top of a magnetic stirrer controller 3. Put a magnetic stirrer inside the bottle 4. Start stirring
81
5. Weigh the chemicals (please see the composition in the table) and put them inside the bottle. DO NOT mix the chemicals. After weighing each chemical clean your spatula so you won‟t contaminate other chemicals. Be sure that your TA is watching you during this process.
6. Mix until all the chemicals are dissolved well 7. Add water to make up 500 ml of medium and mix well 8. Place the cap on the bottle and keep it a little bit loose. DON‟T forget to do
this: otherwise pressure will build up inside, which may cause damage when you take the bottle out.
9. Put the bottle in a tray 10. Autoclave at 121oC for 20 minutes 11. Close the cap 12. After the autoclaving is finished, cool the medium down to room
temperature
b. Growth medium for electricity generation 1. Put approximately 400 ml of DI water in a 1000-mL pyrex bottle 2. Put the bottle on top of a magnetic stirrer controller 3. Put a magnetic stirrer inside the bottle 4. Start stirring 5. Weigh the chemicals (Please see the compositions in Table 2 and Table
3) and place them inside the bottle 6. Mix until all the chemicals are dissolved completely 7. Take 10 clean erlenmeyer flasks 8. Put 100 ml of medium in each of the flasks 9. Close the flasks using aluminium foil. See Figure 18j. Use two layers of
aluminium foil for each flask to ensure there is no contamination. 10. Put the flasks in an autoclavable tray 11. Autoclave at 121oC for 15 mins 12. When the autoclaving is done, take the tray out and cool the medium
down to room temperature.
3.5. Cathode compartment
Fill with a buffer (pH = 7) of the composition shown below:
Table 4. Buffer composition
Components Formula Composition (g/L)
Disodium phosphate Na2HPO4 1.825
Monopotassium phosphate
KH2PO4 0.35
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3.6. Starting the MFC
Make sure you have the following items ready: 7. Assembled and sterilized MFC 8. Sterile growth medium for inoculation 9. Sterile growth medium for electricity generation 10. Disposable syringe and needle 11. Available laminar hood 12. Stock culture 13. 1-ml syringe, needle 14. Buffer 15. 70% alcohol 16. Air pump 17. Disposable syringe and needle
Use the following procedure to start the MFC 1. Clean and sterilize the laminar hood using 70% alcohol 2. Spray some alcohol on the MFC before putting it inside the hood 3. Take the sterilized MFC into the hood. 4. Take the sterile growth medium for inoculation, spray some alcohol on the
bottle, and keep it inside the hood 5. Remove the DI water from both the anodic and cathodic compartments 6. Remove the silicon rubber from the two inlets of the cathode at the top of
the cathode compartment 7. Fill the cathode compartment with buffer 8. Fill the anode compartment with growth medium for inoculation 9. Innoculate the MFC:
a. Take stock culture from the freezer (kept at -85oC) to the hood and let the culture thaw. Do not wait for prolonged times because thawed cells may lose their activity. Generally it takes several minutes for thawing.
b. Take one flask with sterile growth medium to the hood c. Take a 1-mL syringe and needle d. Open the stock culture vial e. Take the culture using the syringe f. Put the stock culture inside the anode compartment through one
opening at the top. You do not need to remove the silicon rubber. Just insert the needle through the silicon rubber and push the syringe.
g. Shake the MFC a little so that the culture is mixed well immediately 10. Take the MFC out from the hood 11. Please DO NOT forget to clean the hood using 70% alcohol and keep all
tools where they belong 12. Pump air into the cathode at a moderate rate 13. Pump air into the anode chamber (for better growth and biofilm formation
on the electrode surface). Keep pumping air only for 24 hrs. You need to
83
stop the air for electricity production: otherwise electrons will be delivered to the oxygen.
3.7. Operation of the MFC in batch mode After starting the MFC the following steps are required for the operation of the MFC.
1. Maintain air flow to the anode compartment for 24 hrs 2. Maintain air flow to the cathode compartment continuously 3. Follow the fill and draw method: Draw 100 ml of cell culture from the
anode side and fill with 100 ml of new medium. Consult with your TA during this fill and draw, because, depending on the experimental plan, you may need to change the fill and draw procedure. a. Put the MFC into the hood b. Remove 100 ml of cell culture from the anode by releasing the opening
at the bottom c. Put the flask of growth medium (100 ml) for electricity generation into
the hood d. Use a syringe to pump in 100 ml of new medium
4. Replace 100 ml of buffer from the cathode every day. You can do this outside the hood
4. Monitoring the potentials of the anode, cathode and MFC and the current of
the MFC We use a data logger to monitor the electrode potentials and current. Consult with your TA about connecting the electrodes to the data logger and computer system.
4.1. Connecting the MFC to the data logger 1. Connect the data logger to the computer (ask TA for assistance) 2. Connect the three electrodes to the three electrical wires 3. Label the wires as anode, cathode and reference 4. Connect the other end of the cable coming from the electrodes to data
logger port #204 (anode), #205 (cathode) and ground (reference), respectively
5. Verify the potentials read by the data logger using a multimeter reading data directly from the MFC ports
6. A graphical representation may be observed on the computer screen if it is connected to the HP data logger.
2. Open the file with „MS Excel®‟ 3. Save as an „xls‟ file. You are ready to analyze the data.
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5. EXPERIMENTAL PLAN FOR CHE 475 We expect the students to spend a total of seven hours and follow the following steps in the lab.
Day 1 (3 hrs): Make the MFC ready
1. Clean the MFC parts 2. Measure and record the surface areas of the anode and cathode 3. Assemble the MFC 4. Prepare the growth medium for inoculation and inoculate it with the bacteria 5. Sterilize the MFC and medium
Day 2. (1 hr): Start the MFC Inoculate the MFC and monitor the open circuit potentials
1. Start the MFC (see starting procedure) 2. Test the initial potential using a multimeter 3. Set the data acquisition system 4. Monitor the potentials continuously every 15 mins: Connect a 1000-Ω resistor
between the anode and the cathode and monitor the anode and cathode potentials with respect to the reference electrode
Day 3. (30 mins): Fill and draw
5. Draw 100 ml of medium from the anode and feed in 100 ml of new sterile growth medium
6. Draw 100 ml of buffer from the cathode compartment and feed in100 ml of fresh buffer
Day 4 (30 mins): Fill and draw
7. Draw 100 ml of medium from the anode and feed in 100 ml of new sterile growth medium
8. Draw 100 ml of buffer from the cathode compartment and feed in 100 ml of fresh buffer
Day 5. Characterize the MFC (1 hr)
9. Characterize the MFC using a polarization experiment (Your TA will help you to run this experiment)
a. Change the setting to read data every 5 seconds b. Disconnect the resistor from the MFC c. Wait until the MFC has a cell potential of 500 mV d. Be sure the resistor box is ready e. Be sure that the resistor box has at least 0 to 10 kohms
85
f. Plan how you will change the resistor: advance planning is encouraged for a quick change of the resistor
g. Tips: The resistor box is operated manually, so be careful while changing the resistors. Practice this one with your TA.
h. Change the resistor every 30 sec. Start from 10K then 9k, 8k…1k, 900, 800…200, 100, 90, 80…20,10, 9, 8…2,1, done!!!
Day 6 (1 hr): Deassembling and cleaning
10. Stop the MFC and data acquisition system 11. Take the medium and buffer out of the MFC 12. Clean the parts with lots of hot water 13. Autoclave all MFC parts other than the membrane 14. Clean the membrane using hot water (don‟t scratch with a sharp object) and
keep it in a 0.1 M NaCl solution Report: For day-to-day operation and data acquisition:
1. Anode, cathode and cell potentials vs. time For characterization experiments: 2. Cell potential vs. current density 3. Power density vs. current density 4. Answers to the test questions
6. SAMPLE EXPERIMENTAL RESULTS
Experiment 1 Applying a constant load and observing for a long time
0
0.1
0.2
0.3
0.4
0.5
0.6
0 200 400Time (hrs)
Cell p
ote
nti
al (V
)
Figure 26. A batch MFC run for a long time. The arrows show the fill and draw times (here fill
and draw was done when the cell potential became very low (around 50-100 mV).
86
A 1000-ohm resistor was connected between the anode and the cathode and the cell potential
was monitored overnight. The MFC was run using Klebsiella pneumoniae using the growth
medium described above. The surface area of the electrode was 47 cm2. The potentials of the
electrodes were measured against SCE. Sample raw data are shown in Table 5.
Table 5. Sample raw data from the HP data logger. Potentials were read every 15 sec. VDC
indicates DC voltage.
Anode Cathode
204(Seconds) 204(VDC) 205(Seconds) 205(VDC)
0.018 -0.05693 0.018 1.47E-01
15.002 -0.05758 15.002 1.47E-01
30.002 -0.05824 30.002 1.47E-01
45.002 -0.05889 45.002 1.47E-01
60.002 -0.05954 60.002 1.47E-01
75.002 -0.06019 75.002 1.47E-01
90.002 -0.06084 90.002 1.47E-01
105.002 -0.0615 105.002 1.47E-01
0
50
100
150
200
250
300
0 200 400Time (hrs)
Po
wer
den
sit
y (
mW
/cm
2)
Figure 27. Power vs. time. This profile looks the same as that of cell potential vs. time. The
1. What voltage did you record with a single lemon? ______ milliamps? _______
2. What voltage did you record with the two lemons connected in series? __________
milliamps? _________
3. What voltage and amperage did you record with the lemons in parallel? ______
milliamps?_______
4. The movement of ___________ from the zinc nail through the wire accounted for the voltage
displayed on the voltmeter.
5. Protons (H+) moved from the nail to the ______________ through the lemon juice. This
suggests that the lemon juice acted as a(n) _________________.
6. In terms of electron flow, explain how two lemons connected in series increased the voltage
of your battery. How was the amperage (the rate of flow) affected?
7. What changes to the lemon, nail, and penny might decrease the voltage?
8. Could a lemon battery power a light bulb?
50
Student Activity/Lab #4 Teacher Answer
Sheet
Transfer of Energy: The Lemon Battery
1. What voltage did you record? Amps? About 0.9 volts, but this will vary. About 5mv.
2. What voltage did you record with the two lemons connected in series? The voltage will
approximately double. The amperage will not change.
3. What voltage and amperage did you record with the lemons in parallel? The voltage will not
change but the amperage will approximately double.
4. The movement of electrons from the zinc nail through the wire accounted for the voltage
displayed on the voltmeter.
5. Protons (H+) moved from the nail to the penny through the lemon juice. This suggests that
the lemon juice acted as an electrolyte.
6. In terms of electron flow, explain how two lemons increased the voltage of your battery. Two
lemons had a greater electron flow than one lemon. The rate of electron flow (amperage) did not
change.
51
7. What changes to the lemon, nail, and penny might decrease the voltage? A less acidic lemon,
a tarnished nail and a tarnished penny could all decrease the voltage.
8. Could a lemon battery power a light bulb? Only a very small light bulb. But theoretically,
enough lemons connected in series could power a light bulb.
Activity/Lab #5
Purpose/General Activity Information:
This activity is the final activity students do prior to doing their final engineering project.
Students are introduced to the final two project questions:
1. Design three ways of powering a calculator using the same electrolyte
2. Design the cheapest, most powerful (most efficient) method to power a
piezo buzzer
Students are given the materials they can choose from and told that each will have a
cost. Students are not told the cost of each item. They then are given one to two class
days to experiment on different variables. Each of these variables is recorded into a
data table. In addition, students will perform one lab write-up for a question that they
want to solve surrounding the projects final question.
Conclusion/Teacher Notes:
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Students can look at the following variables (and probably many more). Bear in mind
that students may bring an electrolyte of their choice to test on since they will have this
freedom in their final project.
Variable Explanation
Material used for the anode
The magnesium strips are the most reactive; therefore, giving the highest current and voltage. This has to do with the energy potential of each material.
Voltage and current are affected
Material used for the cathode
The copper will have the highest affinity for electrons.
Voltage and current are affected
Distance between anode and cathode when using a lemon/fruit
When the distance between the anode and cathode is decreased then the voltage and current increases because there are fewer pulp types of materials getting in the way.
Size (width/length) of cathode
The greater the surface area the current (this may not show due to the membranes in the lemon/fruit.)
Voltage is NOT effected
The material used for the electrolyte, anode, and cathode are the only things that affect voltage
Size of anode (width/length)
Students should find the greater the surface area the higher the current
Voltage is NOT affected
The material used for the electrolyte, anode, and cathode are the only things that affect voltage
Type of electrolyte Students will have a choice of any fruit/vegetable (no juices are allowed). In general the more acidic/basic the greater the voltage
Voltage and current are affected
Type of wire Students will find no major difference between the two
With much, much larger batteries the copper will work out better, but the current is too low here
The type of wire will not effect the voltage
Connecting in Series The voltage will increase (hopefully double)
Connecting in Parallel The current will increase (hopefully double)
The following data table is included for your reference. Students should not see this table.
53
Connection (Series or Parallel)
Wire Type Electrolyte Anode (-) Cathode (+)
Voltage Current
#1 --- Copper Vinegar (50 mL in a 100
mL beaker)
Galvanized Nail 3mm
diameter
Copper Wire
(1 mm diameter)
T1 = .90 V T2 = .96 V T3 = .99 V
T1 = 1.1 mA
T2 = .9 mA
T3 = 1.02 mA
#2 --- Copper Vinegar (50 mL in a 100
mL beaker)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
T1 = .95 V T2 = .94 V T3 = .95 V
T1 = 2.03 mA
T2 = 1.4 mA
T3 = 1.6 mA
#3 Copper Vinegar (50 mL in a 100
mL beaker)
Zinc Strip 1 x 5 cm
Copper Stip
1 cm x 5 cm
1.2 V 1.4 1.6
about equal to current
with galvanized
nail
#4 --- Copper Vinegar (50 mL in a 100
mL beaker)
Galvanized Nail 3mm
diameter
Aluminum Wire 1mm
diameter
T1 = .340 V T2 =
.487 V T3 =
.466 V
T1 = .025 mA
T2 = .022 mA
T3 = .023 mA
#5 --- Copper Vinegar (50 mL in a 100
mL beaker)
Galvanized Nail 3mm
diameter
Penny shiny
T1 = .96 V T2 = .96 V T3 = .95 V
T1 = 2.04 mA
T2 = 1.5 mA
T3 = 1.8 mA
#6 --- Copper Vinegar (50 mL in a 100
mL beaker)
Aluminum Wire 1mm
diameter
Penny shiny
T1 = .42 V T2 = .45 V T3 = .44 V
T1 = .07 mA
T2 = .04 mA
T3 = .03 mA
#7 --- Copper Vinegar (50 mL in a 100
mL beaker)
Aluminum Strip
1.4 cm width
Penny shiny
T1 = .55V T2 = .53 V T3 =
..53 V
T1 = .06 mA
T2 = .06 mA
T3 = .05 mA
#8 --- Copper Vinegar (50 mL in a 100
mL beaker)
Copper Strip
1 cm x 5 cm
Copper Stip
1 cm x 5 cm
T1 = 17.3 mV T2 = 15.1 mV
T3 = 11 mV
T1 =.003 mA
.006 mA
.005 mA
54
#9 --- Copper Vinegar (50 mL in a 100
mL beaker)
Copper Strip
1 cm x 5 cm
Galvanized Nail 3mm
diameter
(--.89 V)
(--.89 V)
(--.74 V)
(-1.6 mA) (-1.3 mA) (-1.5 mA)
#10 --- Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Aluminum Strip
1x5 cm width
.43
.44
.45
.095 mA
.096 mA
.096 mA
#11 --- Copper Vinegar (50 mL in a 100
mL beaker)
Galvanized Nail 3mm
diameter
Old Penny (dull)
.99 V
.98 V
.99 V
2.9 mA 2.2 mA 1.9 mA
#12 --- Copper Lemon (not mashed/rolled)
Galvanized Nail 3mm
diameter
Penny shiny
.93 V
.93 V
.92 V
.18 mA
.16 mA
.17 mA
#13 ---- Copper Lemon (not mashed/rolled)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.95 V
.94 V
.93 V
.29 mA .3 mA
.27 mA
#14 ---- Copper Vinegar (50 mL in a 100
mL beaker)
Galvanized Nail 3mm
diameter
Aluminum Strip
1x5 cm width
.43 V .38 .42
.066 mA
.067 mA
.066 mA
#15 ---- Copper Lemon (not mashed/rolled)
Galvanized Nail 3mm
diameter
Aluminum Wire 1mm
diameter
.37 V
.39 V
.34 V
.018 mA
.016 mA
.018 mA
#16 ---- Copper Lemon (not mashed/rolled)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.9 V .87 V .87 V
.2 mA .19 mA .22 mA
1 cm apart cathode vs. anode
#17 ---- Copper Lemon (not mashed/rolled)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.87 V
.85 V
.86 V
.14mA .14 mA .17 mA
3 cm apart cathode vs. anode
#18 ---- Copper Lemon (mashed/rolled)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.79 V
.81 V
.79 V
.55 mA
.63 mA
.52 mA
1 cm apart cathode vs. anode
#19 ---- Copper Vinegar (25 mL in a 100
mL beaker)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
..98 V .96 V .96 V
1.02 mA .96 mA 1.06 mA
#20 ---- Copper Vinegar (15 mL in a 100
mL beaker)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.91 V
.92 V .9 V
.3 mA .27 mA .27 mA
55
#21 ---- Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.95 V
.94 V
.94 V
5 mA 4.7 mA 4.5 mA
#22 Copper Lemon Juice (plastic
squeeze bottle)
Zinc Strip 1 x 5 cm
Copper Stip
1 cm x 5 cm
1.06 1.02 1.02
7 mA
#23 ---- Copper Lemon Juice (plastic
squeeze bottle)
Aluminum Strip
1.4 cm width
Copper Stip
1 cm x 5 cm
.45 V
.42 V
.42 V
.025 mA
.023 mA
.029 mA
#24 ---- Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Aluminum Rod
.7 cm diameter
.47
.47
.47
.099 mA
.099 mA .1 mA
#25 ---- Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Galvanized Nail 3mm
diameter
.023
.032
.026
.027
.023
.023
#26 ---- Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Penny shiny
.99
.96
.99
5.2 mA 5.2 mA 4.5 mA
#27 ---- Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Copper Wire
(1 mm diameter)
.95
.95
.92
4.2 mA 3.3 mA 3.3 mA
#28 ---- Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Aluminum Wire 1mm
diameter
.57 V .57 .58
.021 mA
.023 mA
.021 mA
#29 ---- Copper Orange Juice Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.94
.93
.94
2.4 mA 2.9 mA 2.3 mA
#30 ---- Copper Orange Juice Galvanized Nail 3mm
diameter
Penny shiny
.99
.97
.96
1.8 mA 2 mA
1.6 mA
#31 ---- Copper Lemonade Frozen
Concentrate 15% lemon
juice
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.93 V
.93 V
.93 V
.3 mA
.4 mA
.3 mA
56
#32 ---- Aluminum Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.95 V
.97 V
.93 V
4.1 mA 4.6 mA 4.6 mA
Aluminum Wire instead of copper
#33 2 in series Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
1.9 V 1.9 V 1.9 V
5 mA 4.9 mA 4.8 mA
#34 3 in series Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
2.3 2.3
2.4 V
5 mA 5.1 mA 4.8 mA
#35 2 in Parallel Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.89 .9
.89
6 mA 5.9 mA 6 mA
#36 3 in Parallel Copper Lemon Juice (plastic
squeeze bottle)
Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.89 V ,9 .9
9 mA 9 mA 9 mA
#37 ---- Copper Lemon Juice (plastic
squeeze bottle)
Magnesium Stip
.4 cm x 5 cm
Copper Stip
1 cm x 5 cm
1.9 V 1.9 V 1.9 V
7.9 mA 9 mA
10 mA 10.3 mA
#38 ---- Copper Vinegar (15 mL in a 100
mL beaker)
Magnesium Stip
.4 cm x 5 cm
Copper Stip
1 cm x 5 cm
1.7 V 1.6 V 1.7 V
2.1 mA 1.7 mA 2.2 mA
#39 ---- Copper Vinegar (15 mL in a 100
mL beaker)
2 Magnesium
Strips .4 cm x 5
cm
Copper Stip
1 cm x 5 cm
1.7 V 1.6 V 1.7 V
3.3 mA 3.3 mA 3.0 mA
#40 ---- Copper Lemon Juice (plastic
squeeze bottle)
3 Magnesium
Strips .4 cm x 5
cm
Copper Stip
1 cm x 5 cm
1.7 1.7 1.7
14.3 mA 13.5 mA 16 mA
12.7 mA 11 mA
#41 ---- Copper Apple Juice Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
1.o 1.0
1.01
5.2 mA 5 mA 5 mA
#42 ---- Copper Apple Juice 1 Magnesium
Strip
Copper Stip
1 cm x 5 cm
1.6 V 1.6
1.67
3.2 mA 3 mA
2.9 mA
#43 Copper Apple Juice Galvanized Nail 3mm
diameter
Aluminum Strip
1x5 cm width
0.42 .46 .46
.06 mA
.06 mA
.06 mA
57
#44 Copper Diet Coke Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.99 V
.98 V
.99 V
.2 mA
.2 mA
.2 mA
#45 Copper Tomato Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
1.01 1.0
.2 mA
#46 Copper Tomato 1 Magnesium
Strip
Copper Stip
1 cm x 5 cm
1.67 1.7
.3 mA
#47 Copper Tomato Galvanized Nail 3mm
diameter
Aluminum Strip
1x5 cm width
.4 V .045 mA
#48 Copper Tomato 1 Magnesium
Strip
Aluminum Strip
1x5 cm width
1.1 1.17
#49 Copper Peach Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
1 .3 mA
#50 Copper Peach 1 Magnesium
Strip
Copper Stip
1 cm x 5 cm
1.5
#51 Copper Peach Galvanized Nail 3mm
diameter
Aluminum Strip
1x5 cm width
0.44
#52 Copper Peach 1 Magnesium
Strip
Aluminum Strip
1x5 cm width
1.09 .4 mA
#53 Copper Lime Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
.99 V
.98 V
.99 V
.1 mA
#54 Copper Lime 1 Magnesium
Strip
Copper Stip
1 cm x 5 cm
1.69 .3 mA
#55 Copper Lime Galvanized Nail 3mm
diameter
Aluminum Strip
1x5 cm width
.35 V
#56 Copper Lime 1 Magnesium
Strip
Aluminum Strip
1x5 cm width
1.1 V
#57 Copper Orange Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
0.95
58
#58 Copper Orange 1 Magnesium
Strip
Copper Stip
1 cm x 5 cm
1.5 V
#59 Copper Orange 1 Magnesium
Strip
Aluminum Strip
1x5 cm width
.97 V
#60 Copper Orange Galvanized Nail 3mm
diameter
Aluminum Strip
1x5 cm width
0.42
Instructional Strategies:
The teacher should observe and help student as needed making sure students are
filling out there data table and that students are examining one variable at a time.
Data Collection:
Students will fill in their data table as they work through the questions/variables they
are testing
Students will perform one experiment design of their own from one of the
questions/variables they feel might affect the lemon batteries power.
Data Analysis:
Students will decide on what variables to test, test this variable using the multimeter,
and record their findings into a data table.
59
Students will decide on what one variable they want to perform an experiment write-
up on and then perform that experiment recording their data into a data table and
then writing a conclusion based on their experimental results.
Evaluation Protocols:
Formative assessment: The teacher will monitor student variables and make sure
that students are finding the correct conclusions. If not, the teacher needs to
determine what went wrong and have students redo the experiment.
Summative assessment: Students will be asked to come up with their own
variables, plan their own investigations, record their experimental data, and come up
with a conclusion based on their data. They will then take this data to solve the final
engineering project:
Design the cheapest (most efficient) way to power a calculator using a lemon
battery.
Worksheet/Handout to be given to Students: (next page)
60
Name: _____________________________
Period: ______
Variables in Batteries
Purpose:
In this activity you will perform a series of experiments to determine how different
variables effect either the voltage or the current. You will record this data into a data
table and use the information in the final two projects:
1. Design three ways of powering a calculator using the same electrolyte
2. Design the cheapest, most powerful (most efficient) method to power a
piezo buzzer
These two engineering problems should focus what experiments you decide to perform.
Keep these in questions in mind as you do the experiments so you can relate your
results to the two engineering issues above.
Also, keep in mind the final projects rules:
1. You may bring any fruit/vegetable you want to use from home/the store.
2. Each fruit/vegetable you use has a cost
3. Bring fruits/vegetables to test. All other materials will be given for you to use. If
you decide not to then you may only use the potatoes.
4. Each material you use will have a cost
5. You may alter the fruit/vegetable as you see fit
61
6. All other materials used in your battery are provided at your table:
Directions:
Using only solid fruits and/or vegetables you are to:
1. Decide on what manipulated variable to test
2. Test the effect that variable has on voltage and current
3. Perform at least two trials per group
4. Record your data into the data table given to you
5. Write a conclusion based on the experimental results.
You also need to come up with one question (manipulated variable) to design an
experiment around. The question will not be given to you. You and your partner need
to decide on a question and then scientifically answer the question through an
investigation.
Materials:
magnesium strips
copper strips
aluminum strips
zinc strips
copper penny
zinc nail
Copper wire
Aluminum wire
Graphite
Vinegar
Aluminum wire
Copper wire
Potatoes
62
Apples
Electrolytes (fruits/vegetables) of your choice from home
Multimeter
Manipulated Variable Tested?
Voltage Current
Manipulated Variable Tested?
Voltage Current
Manipulated Variable Tested?
Voltage Current
63
Manipulated Variable Tested?
Voltage Current
Manipulated Variable Tested?
Voltage Current
64
Manipulated Variable Tested?
Voltage Current
Manipulated Variable Tested?
Voltage Current
Manipulated Variable Tested?
Voltage Current
65
Manipulated Variable Tested?
Voltage Current
Manipulated Variable Tested?
Voltage Current
Manipulated Variable Tested?
Voltage Current
66
List four controlled variables in your experiments:
Hypothesis (prediction) of the investigation results
Materials that includes containers, all measurement devises, and anything else used
Procedure that includes:
One manipulated (changed) variable
One responding (dependent) variable
One controlled (kept the same) variable
Logical steps to do the investigation
How often measurements are taken and recorded
67
Question:
Hypothesis (Prediction):
Materials:
Use the space below to draw a labeled diagram to support your procedure:
68
Procedure:
Data:
69
Based on the data table from your experiment, write a conclusion that;
Answers the investigative question
Includes supporting data
Explain how the data supports your conclusion
Question:
Activity/Lab #6
Purpose/General Activity Information:
This is the first engineering task students need to solve. They must design three
methods of powering the calculator using the same electrolyte in all three solutions.
This will take one class day to complete. All materials are supplied for this activity
including the electrolyte. Students may choose to bring their own if you want.
Conclusion/Teacher Notes:
70
Students will need to connect their battery in series to get enough voltage to power the
calculator. We suggest using the TI 30xA calculator. The Sunway electric calculator
proved to be too easy. Also, to power the calculator the screen must be easily read (not
faint). We suggest using a standard electrolyte and allowing students to alter the
electrodes and any other variables.
Material Series Parallel Electrolyte Wire Type Anode (-) Cathode
(+) Power/Description
Sunway Electric Calculator
- (SK - 8819B) - 1.5 V - 1.6 mA
Lemon Juice 40 mL in a
100 mL beaker
Copper 2 magnesium
stips
Copper Stip
1 cm x 5 cm
Power
Sunway Electric Calculator
- (SK - 8819B) - 1.5 V - 1.6 mA
Lemon Juice 40 mL in a
100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
Power
Sunway Electric Calculator
- (SK - 8819B) - 1.5 V - 1.6 mA
2 Lemon Juice 40 mL in a
100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power
Sunway Electric Calculator
- (SK - 8819B) - 1.5 V - 1.6 mA
2 Vinegar 40 mL in a
100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power
TI 30xA Calculator
~ 3 V
Lemon Juice 40 mL in a
100 mL beaker
Copper 2 magnesium
stips
Copper Stip
1 cm x 5 cm
Power, but very faint numbers (will
do calculations)
TI 30xA Calculator
~ 3 V
Lemon Juice 40 mL in a
100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
Power, but very faint numbers (will
do calculations)
TI 30xA Calculator
~ 3 V
Vinegar 40 mL in a
100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
Power, but very faint numbers (will
do calculations)
TI 30xA Calculator
~ 3 V
Vinegar 40 mL in a
100 mL beaker
Copper 1 magnesium
strip
Penny Power, but very faint numbers (will
do calculations)
71
TI 30xA Calculator
~ 3 V
Vinegar 40 mL in a
100 mL beaker
Copper 2 magnesium
stips
aluminum strip
No power
TI 30xA Calculator
~ 3 V
Lemon Juice 40 mL in a
100 mL beaker
Copper 2 magnesium
stips
copper wire only
No power
TI 30xA Calculator
~ 3 V
2 Lemon Juice 40 mL in a
100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
Power
TI 30xA Calculator
~ 3 V
2 Lemon Juice 40 mL in a
100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power, very faint numbers and won't
calculate
TI 30xA Calculator
~ 3 V
3 Lemon Juice 40 mL in a
100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power
TI 30xA Calculator
~ 3 V
2 Vinegar 40 mL in a
100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power, very faint numbers and won't
calculate
TI 30xA Calculator
~ 3 V
3 Vinegar 40 mL in a
100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power
TI 30xA Calculator
~ 3 V
2 Lemon Juice 40 mL in a
100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
power
TI 30xA Calculator
~ 3 V
3 Salt Water Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
TI 30xA Calculator
~ 3 V
2 Apple Juice 40 mL in 100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power, but very faint numbers (will
do calculations)
TI 30xA Calculator
~ 3 V
3 Apple Juice 40 mL in 100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power
TI 30xA Calculator
~ 3 V
Apple Juice 40 mL in 100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
No power to a very faint number that won't calculate
TI 30xA Calculator
~ 3 V
Apple Juice 40 mL in 100 mL beaker
Copper 2 magnesium
stips
Copper Stip
1 cm x 5 cm
No power to a very faint number that won't calculate
TI 30xA Calculator
~ 3 V
Diet Coke 40 mL in 100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
Power, but very faint numbers (will
do calculations)
72
TI 30xA Calculator
~ 3 V
2 Diet Coke 40 mL in 100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
Power
TI 30xA Calculator
~ 3 V
Tomato Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
Power, but very faint numbers (will
do calculations)
TI 30xA Calculator
~ 3 V
3 Lime - tomato - peach
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Power
Instructional Strategies:
Students will have free reign to work on their own solutions. They should use the data
table they put together to guide their thinking. The teacher needs to monitor and help
as needed.
Data Collection:
Students will draw a labeled diagram of their three answers.
Data Analysis:
Students will utilize their data sheet from the previous activity to test different solutions
for powering the calculator. They will then draw a diagram of their three solutions
labeling the different materials they used.
Evaluation Protocols:
73
This is a formative assessment. The teacher should monitor student interactions and
help where needed.
Worksheet/Handout to be given to Students: (see next page)
Name: ______________________________
Period: _____
SPUD POWERED CALCULATOR
Purpose/Problem: You need to devise three different methods to power the calculator
using potato(es).
74
Rules:
1. You may not alter the potato in any way except to put your electrodes into the
potato
2. You may use only the potato as the electrolyte
3. The only source of power for the calculator is the potato
Materials:
Potatoes
TI 30xA calculator
Copper Wires
Potatoes
Magnesium strips
Galvanized Nails (Zinc coated)
Zinc Strips (3 x 5 cm)
Aluminum strips
Aluminum wires
Copper wires
Copper strips
Penny
Question: Answer the following questions. Draw a diagram of each of your three solutions
Label each part: o What did you use for the wire? o What length of wire did you use? o What did you use as the cathode? o What is the length and width of the cathode?
75
o What did you use as the anode? o What is the length and width of the anode? o What is your electrolyte? o How much electrolyte did you use? o Label any other important features
Label the direction of electrons as it flows through your circuit. Labeled diagram of solution #1: Labeled diagram of solution #2: Labeled diagram of solution #3:
Activity/Lab #7
Purpose/General Activity Information:
76
This activity is the final culminating engineering project. Students were introduced to
this problem in the last activity/lab. Students will most likely need two class days to
complete; although, some students may finish on the first day. The culminating
engineering question is:
Design the cheapest, most powerful (most efficient) method to power a piezo
buzzer
Conclusion/Teacher Notes:
There are a variety of correct answers students could come up with. Students need to
follow the rules and are allowed to bring an electrolyte of their choice. They should use
their data table to help them devise their solution. Following is a table that may help
you:
Material Series Parallel Electrolyte Wire Type
Anode (-) Cathode (+)
Power/Description
Piezo Buzzer - (Radio Shack) - 3V - 28 V - 5 mA
Lemon Juice
40 mL in a 100 mL beaker
Copper 1 magnesium
strip
Copper Stip
1 cm x 5 cm
Buzzed
Piezo Buzzer - (Radio Shack) - 3V - 28 V - 5 mA
Lemon Juice
40 mL in a 100 mL beaker
Copper Galvanized Nail 3mm
diameter
Copper Stip
1 cm x 5 cm
Low Buzzer
Piezo Buzzer - (Radio Shack) - 3V - 28 V - 5 mA
Lemon Juice
40 mL in a 100 mL beaker
Copper Galvanized Nail 3mm
diameter
aluminum strip
No buzz
77
Piezo Buzzer - (Radio Shack) - 3V - 28 V - 5 mA
Lemon Juice
40 mL in a 100 mL beaker
Copper 1 magnesium
strip
aluminum strip
Low Buzzer
Piezo Buzzer - (Radio Shack) - 3V - 28 V - 5 mA
2 Lemon Juice
40 mL in a 100 mL beaker
Copper 1 magnesium
strip
copper High Buzzer
Piezo Buzzer - (Radio Shack) - 3V - 28 V - 5 mA
2 Lemon Juice
40 mL in a 100 mL beaker
Copper 1 magnesium
strip
aluminum strip
Low Buzzer
Piezo Buzzer - (Radio Shack) - 3V - 28 V - 5 mA
2 Lemon Juice
40 mL in a 100 mL beaker
Copper Galvanized Nail 3mm
diameter
aluminum strip
Very low buzz
Equipment/Materials:
Potatoes (for students that do not bring their own electrolyte)
Piezo buzzer
Copper Wires
Magnesium strips
Galvanized Nails (Zinc coated)
Zinc Strips (3 x 5 cm)
Aluminum strips
Aluminum wires
Copper wires
Copper strips
Pennies Instructional Strategies:
The teacher should observe student interactions and let students solve the question
with minimal teacher involvement.
Data Collection:
78
Students will diagram out their final solution and give reasons for each choice they
made. This handout is provided to the student.
Data Analysis:
Students will share this information and the reasons behind their decisions in a post-
project write-up.
Evaluation Protocols:
This is a formative assessment. Students should have a basic understanding learned
through experimentation. They now apply what they learned to solve the engineering
problem.
Worksheet/Handout to be given to Students: (see next page)
Name: ____________________________
Period: _____
79
FRUIT POWER!!
Purpose:
Design the cheapest, most powerful (most efficient) method to power a piezo buzzer
Rules:
1. You want a loud buzzer, but you also want it to be cheap. Just because your
solution is the loudest does NOT mean that your solution will be the winner of the
competition.
2. You may not remove the piezo buzzer or the wires from their location on the
wood board.
3. You may not use any other materials besides what is provided at your table and
the fruit(s) you bring.
4. You may provide your own solid fruit/vegetable from home, or use the potatoes
provided.
5. You may modify the fruit/vegetable in any way you choose.
Materials/Cost:
Aluminum Strip $1.50 / 5 cm
Aluminum Wire $1.00 / 5 cm
Copper Strip $2.50 / 5 cm
Copper Wire $2.00 / 5 cm
Electrolyte/Fruit or Vegetable $2.50 / fruit or vegetable
Galvanized Nail $1.75 each
80
Magnesium $5.00
Penny $2.25 each
Zinc Strip $2.00
Pre-Project Question:
What is the voltage of the Piezo buzzer:
_______________________________________
What is the current of the Piezo buzzer:
_______________________________________
Final Plan: (answer on next page)
Draw a diagram of your final solution
Label each part: o What did you use for the wire? o What length of wire did you use? o What did you use as the cathode? o What is the length and width of the cathode? o What did you use as the anode? o What is the length and width of the anode? o What is your electrolyte? o How much electrolyte did you use? o Label any other important features
Label the direction of electrons as it flows through your circuit Draw labeled diagram here:
81
Explanation of Final Plan: 1. Wire:
Identify what type of wire you chose to use. Explain why you chose to use this for wire.
Identify the length of wire you chose to use. Explain why you chose to use this length.
2. Cathode:
Identify the material you built your cathode out of. Explain why you chose this material.
Identify the width and length of your cathode. Explain why you chose this width and length.
82
3. Anode:
Identify the material you built your anode out of. Explain why you chose this material.
Identify the width and length of your anode. Explain why you chose this width and length.
4. Electrolyte:
Identify what you used as your electrolyte. Explain why you chose to use this for your electrolyte.
How much electrolyte did you use. Explain why.
5. Series:
Identify if you connected your battery in series.
If you connected your battery in series identify how many series you used
Explain why you did this.
83
6. Parallel:
Identify if you connected your battery in parallel.
If you connected your battery in parallel identify how many parallels you used
Explain why you did this.
Final Note to teacher: Bring students together and put together a data table of what works to increase voltage
and current:
Description Increased Voltage Increased Current
84
References:
Batteries. (September 2004). Kamenny Kruh Stone Circle: An environmental education
program of the Prague Endowment Fund. Vol 54, issue no 1.
Brain, M. How Batteries Work. Retrieved July 26, 2006, from
http://home.howstuffworks.com/battery.htm
Brodd, J.B. & Winter, M. (2004). American Chemical Society. Chemical Review v.
104 pg 4245-4269.
Carboni, G. (1998). Experiments in Electrochemistry. Retrieved July 25, 2006, from